3,452. Attending an Unlearned Friend's Party

Hilchos Maaser 10:6

A learned person may not serve as a waiter at the banquet or party of an unlearned person unless everything has been tithed and rectified under his supervision. Therefore, if you see a learned person serving at the banquet or party of an unlearned person, you can assume that the food has been properly tithed. If we see a learned person eating with an unlearned person, we can’t assume that the food was tithed because the learned person might have mentally made the necessary conditions.

Hilchos Maaser 10:7

Just as one may eat with an unlearned person and rely on his mental conditions, he must likewise stipulate regarding what his son eats; this is so even if the son is elsewhere. He need not stipulate regarding anyone other than his son even if that person is with him at the banquet. Therefore, if the son of a learned person is at a banquet with an unlearned person, we can’t assume that the food was tithed because the learned person may have stipulated regarding his son’s food.