951. Eating in a Friend’s Succah

135:13 If a person goes to a friend’s succah and eats there a portion that must be eaten in a succah, he must recite the bracha “leishev basuccah.” This is true even if he visited his friend’s succah in the middle of a meal he was having in his own succah (that is, unless he had in mind that he would do this when he recited the bracha in his own succah – Mishnah Brurah 639:48).

135:14 If one forgot to recite “leishev basuccah” and he remembered in the middle of the meal, or even after he finished eating, he must recite the bracha. This is because sitting there after eating is also a mitzvah. (If he hasn’t already bentched, he should recite the “leishev basuccah” and then eat a little bit more – MB ibid.)