Gemara

(Rav): One may not put bread in a waiter's mouth unless he knows that the waiter washed.

Avodah Zarah 6a: Perhaps the Isur to do business with a Nochri before his festival is lest a Yisrael sell something that he will offer to idolatry, and the Yisrael transgresses "v'Lifnei Iver Lo Siten Michshol" (by enabling him to sin). If the Nochri already has an animal, 'Lifnei Iver' does not apply.

Question (Beraisa - R. Noson): "V'Lifnei Iver" forbids extending wine to a Nazir or a Ever Min ha'Chai (a limb from a living animal) to a Nochri.

Even though the Nazir or Nochri could take it himself, one who gives him transgresses!

Answer: The case is, the Nazir or Nochri is on the other side of a river, and he cannot reach it himself.

Support: The Beraisa mentions extending it to him, but not 'giving' it.

Mishnah (Demai 3:1): One may feed Aniyim or guests Demai;

We collect (food) Stam and distribute Stam, and one who wants to tithe will tithe.

Rishonim

Rif (Berachos 42a): Rav forbids giving bread to a waiter unless he knows that the waiter washed.

Gra (OC 169:4): The Rif's text did not say 'put in his mouth.'

R. Yonah (Berachos 42a DH Lo): Some learn from the Isur to give bread to a waiter unless he knows that he washed, that one may give to eat only to one whom he knows that he will bless. I disagree. Since he does so for the Mitzvah of Tzedakah, it is permitted.

Rebuttal (Beis Yosef OC169 DH v'Chasav): Also giving to a waiter is somewhat of a Mitzvah, yet we permit only if he knows that he washed!

Rosh and Ran (Shabbos 1:1 and 1a DH u'Makshu): Even when a minor eats Neveilah, Beis Din must stop him, all the more so one may not help an adult to sin, even if he could sin by himself.

Perush ha'Mishnayos (Demai 3:1): Since Aniyim may eat Demai, why do we say that one who wants to tithe will tithe? He is an Oni. He need not tithe! The Heter of Aniyim is for one meal, to make it easier to give Tzedakah. However, when we distribute the Kupah (Tzedakah box), and the Oni gets a considerable amount, he must tithe it. The Mishnah said 'one may feed Aniyim Demai'. It did not say that Aniyim may eat Demai.

Rambam (Hilchos Ma'aser 10:13): If a Chaver (one who fulfills all laws of Ma'aser) doctor was feeding a sick Am ha'Aretz (without is suspected not to tithe) the Choleh's food, he puts it into the Choleh's hand, but not into his mouth. If it is the doctor's food, he may not put it even in his hand. Likewise, if he is sure that the Choleh's food is not tithed, he may not put it even in his hand.

Kesef Mishneh: This is from the Yerushalmi. Chachamim did not forbid putting the food in the Choleh's hand, for it is the Choleh's food and it is a Safek and he could take it himself and the doctor did not nothing. One may not put Vadai Tevel in his hand, for this is helping a transgressor. Similarly, if it is the doctor's Demai, a Chaver may not feed someone food that was not Vadai tithed.

Poskim

Shulchan Aruch (OC 163:2): If Ploni feeds (bread to) another (David), Ploni need not wash. David must wash, even though Ploni puts food in his mouth and David does not touch it.

Rema: One may not feed one who did not wash, due to Lifnei Iver.

Source: R. Yonah (Berachos 42a DH Ochel, cited in Beis Yosef DH u'Mah).

Magen Avraham (2): It seems that this is only if the food belongs to Ploni. If it is David' food, it is permitted, for David could take it himself, unless he is on the other side of the river. Perhaps in any case it is forbidden due to helping transgressors.

Mishnah Berurah (11): If it is a Safek whether he washed, one may feed him. Certainly, if the eater is poor one may be lenient about a Safek. In any case, if one is unsure it is good to tell him to wash.

Kaf ha'Chayim (17): In a place where they sell food and people buy and eat without washing, the seller did not transgress Lifnei Iver.

Shulchan Aruch (169:1): It is Midas Chasidus to give to a waiter to taste every food (even if it does not have a smell). One may not give bread to a waiter unless he knows that he washed.

Beis Yosef (DH u'Mah): The Tur connotes that the Beraisa forbids not only putting into his mouth. One may not put even into his hand.

Bach (DH Asur): The Beraisa teaches a bigger Chidush. One might have thought that one may put it his mouth, for there is no concern lest he touch it with his hands. Rather, even in such a case the eater must wash. Perhaps the Beis Yosef explains that when he put it in his hand, one might have thought that the waiter will save it and eat it later. The Beraisa forbids because presumably he will eat it immediately.

Magen Avraham (5): The Gemara and Rashi connote that we are concerned only for a waiter, for he is distracted and he might forget to wash. However, R. Yonah understands that we are concerned for anyone.

Mishnah Berurah (9): We are not concerned for someone other than a waiter unless we see that he intends to eat without washing. Some are stringent. Surely, for the sake of Tzedakah one may be lenient, like the Rema in Sa'if 2.

Kaf ha'Chayim (11): The Shulchan Aruch connotes that one may give only to one whom he knows washed. However, if the eater is established to be an important person, one may assume that he washed.

Shulchan Aruch (2): One may give to eat only to one whom he knows that he will bless.

Rema: Some are lenient if he gives to an Oni for Tzedakah.

Bach (DH Kasav): When one gives to one who did not wash, he transgressed Lifnei Iver. If he gives to an Oni who will not bless, the giver does not transgress at all when he gives to him. He does a Mitzvah of Tzedakah! Even if one knows that the Oni will not bless, he should not be Batel from Tzedakah due to this.

Taz (3): R. Yonah holds that the Mitzvah of Tzedakah is Vadai, and it is a Safek whether the Oni will eat without washing, and a Safek does not uproot a Vadai. The Beis Yosef rejected this, for also giving to a waiter is a Mitzvah. I say that a waiter is humble and will not resent being told to wash, but someone else might be offended and refuse to accept, and the Mitzvah of Tzedakah will be lost.

Magen Avraham (6): It seems that if one knows that he will not bless, it is forbidden.

Mishnah Berurah (11): We are more lenient for Tzedakah. Giving to a waiter is not Tzedakah. One gives because the waiter serves him. One may not give Tzedakah if the Oni surely will not bless, unless this is due to Ones.

Kaf ha'Chayim (15): If he will not bless due to Ones, one should give to him something that does not obligate washing and Birkas ha'Mazon.

Kaf ha'Chayim (13): The Levush says that the lenient opinion allows relying on "no mishap will occur to a Tzadik." Olas Tamid says that we are lenient for the sake of the Oni's livelihood. Lechem Chamudos says that those who are lenient for Tzedakah when there is concern lest he not bless, even though Birkas ha'Mazon is mid'Oraisa, all the more so they are lenient when the concern is not washing, which is mid'Rabanan. If one is unsure about the Oni, it is best to tell him to was and bless before and after, or to give to him Peros or money.

Kaf ha'Chayim (14): The Shlah says that we do not refrain from giving Tzedakah to one who does an Aveirah, unless he is a total Rasha. One need not give Tzedakah to one who transgresses Shabbos or transgressed to anger Hash-m.