This happened last night. I'd had a long day and was sad/tired, he'd had a long day and was just tired, she was staying with us and had had (by her own admission) a lovely day of shopping.
I had an argument with DH over something trivial, where we both did our share of stomping. She looked on in horror. I then ate dinner with her while he settled the kids upstairs and she kept on saying 'oh isn't it DREADFUL' in that way people do when they're loving the drama. I tried to get her to drop the subject but she kept com7ng back to it. Finally DH came back down and we settled things amicably. But she kept on saying to him (over his dinner) the same 'dreadful' nonsense. Eventually he lost his patience and had a proper go at her.
He told her she was a poisonous old baggage, that she has a sad lonely existence, that she has driven a wedge between him and his brother - lots of stuff like this, including an accusation that she was just stirring because she loves drama and causing discord (I happen to agree with him). I kept trying to stop them both from speaking but I didn't manage to stop them before enough damage had been caused.
She had a fit of hysterics, seemed really upset and frightened by the fact that SHE was the target ('I've seen him talk to girlfriends like that, but never ME'
) and basically insisted on leaving - I had to call her a cab and book her a room elsewhere ('oh I'm so FRIGHTENED, please help me').
Anyway. I have told DH that whilst I understand his annoyance - and I do, as I think he was entirely correct - I don't approve of him shouting at her and that I think his behaviour was poor and that he owes her an apology for shouting. He agrees (a bit reluctantly). He lets rip like this very rarely and i make sure he gets read the riot act immediately afterwards, so he's not just a wanker.
She has sent us a message of 'you nasty cruel bastards' overnight, which seems unfair to me as I was trying to be nice to her.
So. Would you do anything in this situation, or just leave DH to it? I'm worried that he has 40 years of frustration clouding his judgement.
Thanks for any advice and please excuse the typos below, my stupid tablet hates me 
.').