Here's the thing - I can't tell you how many times I've read on here that someone else's rudeness does not justify you being rude in return.
It doesn't matter if the elderly aunt was rude to not invite the girlfriend. It really doesn't. What matters is that she was specifically not invited and Larry brought her anyway. There is not any situation where it is appropriate to bring an uninvited guest, and especially after it was confirmed by several people that she was in fact not invited. It wasn't a misunderstanding - he knew she was not invited and brought her anyway. That is rude by anyone's definition of etiquette.
Was he in a difficult place? Of course. But he had two etiquette-approved choices (come without the girlfriend for a few hours or decline the invitation) and he chose neither.
I agree with bolded, but not in quite the same say.
I think the first rudeness was Larry's.
If he thought his girlfriend was so important (it apparently wasn't the opinion of his own father or the OP's friend, who knew the specifics of Larry's life), he should have gone politely to his aunt and explained the situation.
And then he should have lived with the answer.
The *second* rudeness was the aunt's.
Lowspark put it well:
I'm still on the side of the Aunt as far as getting to decide who she wants to invite. It's Larry's perogative to either come alone or skip the event, but bringing the uninvited gf was rude. No question.
However, with the update, I think the Aunt went haywire berating Larry within earshot of the rest of the company. That is rude. She had two choices (as we've discussed at length in some other recent threads): Graciously accept the gf as a guest and treat her as such or (difficult but it can be done) turn them both away at the door.
I don't see yelling at Larry as an option, whether it was in private or not. She could have taken him aside quietly and said, sorry GF wasn't invited and you'll both have to leave but that's not what she did.
I agree. Well, I think in private she could yell at Larry; he's not a stranger to her, and so I think you can yell at people you have an actual relationship
with. You might tick them off, but that's all a relationship
So here's the score as I see it:
Aunt: -1 for yelling at Larry, -1 for doing it so everyone could hear = -2
Larry: -2 for bringing uninvited GF, (-1 because it was rude to the Aunt and -1 because it was rude to the GF)
GF: -1 for staying for dinner knowing she wasn't invited, but I give her a pass because I can imagine not really knowing what to do in such a situation.
I'd modify the score:
Aunt: -1 for yelling at Larry where everyone could hear
Larry: -2 for bringing the girlfriend, just because it's such a BIGGER rudeness and was done with such forethought (after warnings from two people); -1 for being rude to girlfriend.
GF: 0; I don't think she should have left after the whole "she wasn't invited" thing unless Larry initiated it.