You hold that Chalav Yisrael is not necessary, relying on the psak of Reb Moshe Feinstein and the Chazon Ish and many other poskim. You have a guest in the house who does not hold like these poskim, who scrupulously avoids what we call Chalav Stam. This guest wouldn't eat food cooked in a pot that had been used for Chalav Stam.
You've run out of Chalav Yisrael. It is difficult to procure a new bottle. You think to yourself, hmmm, I still have the empty bottle of Chalav Yisrael. Maybe I'll just pour the Safeway Supermarket milk, that has a hechsher but is not Chalav Yisrael, into the Chalav Yisrael bottle. Presto! Chalav Yisrael!
This is clearly prohibited. By putting the milk into a container labeled Chalav Yisrael, I am goneiv da'as, I am intentionally misleading my guest. So let's refine the hypothetical.
In my city, all we get is Lubavitcher Shechita. My friends, Lubavitchers, only eat Lubavitcher Shechita. I go to Costco, and, as occasionally happens, they're selling kosher meat, but it's Alle, Satmerer Shechita. I invite my Lubavitcher friends to supper. They don't ask any questions, because all the meat in the city is fine. They know nothing about the anomalous Costco sale. Can I serve them the Satmerer Shechita? They certainly wouldn't eat it if they knew. Do I have to tell them? From my halachic perspective, I am not causing them to "stumble," because the meat is 100% kosher. I am not causing them any harm, and I am not causing them to do anything wrong. Why should I care that my guests hold that it's assur? Perhaps it's akin to a person that has some irrational phobia about a food, and I put it into the meatloaf without telling him. Or if he knew that I dropped something on the floor and then thoroughly washed it, he would never eat it. So I don't tell him, and no harm is done. Shelo befanav michzi chazya(Chulin 71a)! The fact that he has an irrational dislike for a food is meaningless to me, and if he eats it, of no consequence to him.
I'm not talking about parties that have a special relationship of trust. I'm talking about independent parties.
To put it more carefully, here are the questions.
Without verbal misrepresentation, but where the other person does not think to ask about what I am giving him:
Can I give him something I hold is muttar if he holds it's assur.
Does it matter if it is an issur deoraysa or derabanan.
Does it matter if the other person agrees it's muttar but as a personal chumra never eats what I'm giving him.
Many readers will react negatively to the question. What kind of person would do such a thing!!! Shame on you for even asking! My Chashuveh Rebbitzen said a profound truth. What happened to Hillel's rule (Shabbos 31a)? D'alach sani, lechavrach lo sa'avid. If the tables were turned, it certainly would be sani li. I would not just be disappointed, I would be very upset to learn what had happened. Hillel says that the yesod of the whole Torah is that it is wrong for me to disregard my friend's expectations. So this is not just unethical: according to Hillel, it is an affront to the foundation of the Torah.
But that's what Torah is all about. We need to know exactly if and why it's assur, whether it's an issur or a minhag chasidus. There are enormous practical differences. If you're under stress, if you realize there's a problem, and being forthright would result in monetary loss or terrible embarrassment, then the identification of the problem will make the evaluation of the right path clearer. If we're talking about an issur de'oraysa of lifnei iver, then too bad for you. If it's a matter of impropriety, then you have to weigh the cost to you and to your guest.
Is this muttar or assur?
If it is muttar, is it moral or immoral? Yes, of course, it sounds terrible. But is it truly unethical?
If it is unethical, is it reprehensible or merely improper?
If it is assur, is it assur under lifnei iver, or arur mashgeh, or gonev da'as?
(By the way, in the specific Chalav Yisrael case there's another cure. If there's a little milk left over in the Chalav Yisrael container, you can pour in the other milk slowly and rely on Kama Kama Bateil. For you, there's obviously no problem of ein mevatlin issur lechatchila. Of course, there might be a min be'mino lo bateil problem for you, according to the Ran in Nedarim. But for your guest, that won't be a problem since he holds the new milk is assur.)
When I moved into this house twenty four years ago, my son wandered over to the neighbor, and she offered him a piece of gum. He asked her, is it kosher? She answered "It's kosher enough." Baruch Hashem, my son, who, when even younger, ate a dead wasp off a windowsill in shul on a dare, brought it home and asked me if he could eat it. Of course, it was chazir treif. Wrigley, as a representative wrote to me around twenty years ago, sometimes uses emulsifiers that are derived from animal fat. I've never been able to find out whether this woman is a non-Jew married to a Jew, or a totally assimilated Jew, but you don't have to be Jewish to be a mechasheifa.
I was upset by her callous indifference to our religious beliefs. But our first hypothetical is different. Even if it's assur, it's an issur derabanan, and it could be argued (Nesivos 234:3) that a person that does an issur derabanan inadvertently is totally blameless, so I've done him no harm at all. And in the second hypo, his insistence on Lubavitcher shechita is a chumra be'alma, not an issur. Unlike my shikseh neighbor, I have respected poskim that are telling me that it's muttar ahl pi dinei Torah. Why should I be concerned that his poskim asser for him? And she lied. In the hypo, nobody is lying.
Yevamos 14b, Beis Shammai and Beis Hillel on Tzaras Erva, Lo Nim'ne'u
Chulin 111b, Rebbi Elazar hava ka'im
Sukka 10b, Reb Chisda
It is best to look at the Gemaros inside, because doing so will help you form your own opinion. I think each and every one of them is important in thinking about this question. In any case, here's the last half of the discussion.
I think the most important makor for this question is in Chulin 111, but the richest source of marei mekomos is the Gemara in Yevamos. Because Harav Aharon Jofen's notes on the Ritva on 14b 118-122 bring down everything that has every been said on the topic- but, strangely, not the Gemara in Sukkah.
Yevamos 14b, Beis Shammai held that the co-wife of an erva is included in the parsha of yibum. Therefore, Beis Shammi allowed such a yevama to marry her late husband’s brother. Beis Hillel holds that such a marriage is absolutely prohibited, and the offspring of such a yibum would be a mamzer. The result of this disagreement is that there were people in the Beis Shammai neighborhoods that were kasher according to Beis Shammai but Mamzeirim according to Beis Hillel. The Gemara says that Beis Shammai used to tell Beis Hillel about all such children so that they could avoid marrying them. This indicates that where you hold something is muttar, but another person holds it is an issur deoraysa, it is proper to tell them about it. On the other hand, it is possible that Beis Shammai only told Beis Hillel about the problematic children because if they didn't, Beis Hillel would not marry any children from Beis Shammai at all, out of concern that they might be a result of such a union and passul.
The Ritva in Yevamos says that where I hold it's muttar, and the other holds it's assur midorasa, there’s a din of Lifnei Iveir to personally hand him the food. But (see there note 118 from Harav Aharon Jofen) the Pri Chadash OC 496 Issur Ve'hetter #23 says that it's not lifnei iver, but it's ugly. The Ran in Yevamos holds there cannot be lifnei iveir if you hold it’s not assur.
The Meiri there says it is not obligatory to inform the guest, and therefore the guest can only rely on being told where the host is muchzak bechasidus, he is known to be a particularly honest and upright person.
The Mordechai in Yevamos, and the Ohr Zaru’ah in siman 603, and others, all brought in Rama YD 119:7, say that it is the hosts responsibility to inform the guest, and that the guest can rely on the host’s being forthright with him.
But all these are discussing a case of a machlokes whether something is assur midoraysa or muttar. The hypothetical involves a case of issur derabanan. Certainly it would be a different matter if it involved a 'chumra be'alma.'
The Aruch Hashulchan in the recently printed volume on Nedarim, discusses this briefly in 214:34. He just states that if A holds something is muttar, and B holds it’s assur, A may not serve it to B.
The Ksav Sofer in his teshuvos OC 64 , based on the Sha'ar Ha'melech (Hilchos Ishus 7:12) writes that if one person holds that a certain practice is prohibited and he helps another person who maintains that the practice is permitted to do that act, he is over on "Lifnei Iver." Therefore, in the case of a person who holds that smoking on Yomtov (or the entire year) is assur, he may not buy cigarettes for another person because of lo saamod ahl dahm rei’echa, and on Yomtov he must treat cigarettes as Muktzah. If, however, you hold lehalacha that smoking is muttar, but you personally prefer to be machmir, then he may light a cigarette for someone else who smokes the whole year and on yomtov too. It would seem to me that the Ksav Sofer’s logic would apply in the converse case, where you are meikil and the other person has a personal chumra, that you would be allowed to give him the thing that you hold is muttar.
So, although the Ksav Sofer might imply the contrary, for all practical purposes the question has been decided by the Rama in YD 119. The host is obligated to inform the guest, even if it involves only the guest's chumra. If he doesn’t, then he’s a lowlife, but the Rama holds he's not oveir on lifnei iver.
Eli points out that the Shach there, SK 20, seems to hold like the Maharlbach that there is a real possibility of lifnei iveir where the guest holds it's an issur gamur.
I was thinking that our passuk might apply to him. Mashgeh Iveir would seem to apply to any breach of trust, even where the ‘iveir’ simply assumes the other person will take his personal standards and desires into consideration. And the word 'derech' (Brachos 10a) usually means an optional activity, not something connected to mitzvos and issurim, and Chazal apply this din to "eitza she'eina hogenes lo,' advice that is not good for him. But after more thought, I doubt it. First of all, 'Arur' is a pretty heavy punishment for a minor infraction. Second, Mashgeh sounds more active than our hypothetical. And third, unlike makleh aviv, where Reb Meir Simcha says it means showing disrespect even where the parent is mochel, nobody uses this passuk to say my pshat.
UPDATE NISSAN '72: I just saw the Chazon in in Krisos 12b, (DH Sham Lo Amra) regarding a person who knows something is muttar, and two witnesses testify it is assur: the Gemara there says, according to one pshat in Rashi, that he may eat it in private. The Chazon Ish adds that he may not give it to others.