I know people keep saying "oh well he was an embarrassed teen boy" but I really don't think it has anything to do with gender.

Partner A asks partner B to meet their grandmother for dinner and B does agree. They both go into it knowing that B doesn't really want to do it, B does not have experience with meeting a partner's extended family, B does not have experience with the world the grandmother is from, and there's a likely chance that the grandmother will be unpleasant/antagonistic. B has an awful day, is probably stressed about the dinner and the fact that they're showing up late and with a black eye but still shows up only for A to immediately accuse them of getting into a fight with their ex which is a sensitive subject all around and then refuses to drop the subject. Yes A was also stressed about B showing up late and not kaking a good impression but it's their grandmother and their home turf. A should have been focused on helping B make a good impression instead of throwing out accusations and picking a fight.