I see it differently - he is acting as another woman's date for a night. I can understand why that would make someone uncomfortable especially since he didn't bother to check in with her first, not to ask permission but to explain and see how it would make her feel. Instead he just went and announced it like it was a done deal and depending on how long they've been dating (I guess seriousness would trump length though), that's not cool.
That's true - I would have like to know ahead of time with some explanation, rather than it being sprung on me. I don't expect to have any say in it but would like to be informed. 'Honey, for many years, I've accompanied friend to her Christmas party. I'd like to continue to do that, if you're OK with it.'
But a partner who told me to choose between him and my friends? Don't let the door hit you on the way out.
Not in my world. I don't ask Gish for permission to do things, and i do not expect him to, either. It has worked for 10+ really happy years so far. As i have said before, though, i think we are probably a lot more ... liberal? ... than most, so what works there may not work for everyone.
I am quite sure we have caused others some brain hurty moments now and then.
It's worked for 3 years for us, too.
Me: Hey, I'm going to a party with Male Friend on the 8th, it's kind of a tradition.
Him: Oh, I thought we'd go see a movie that night, but it's OK.
Me: Do you want to go see it tonight instead?
Him: Sure.
OR
Me: Hey, I'm going to a party with Male Friend on the 8th, it's kind of a tradition.
Him: I'd rather you not.
Me: OK, why? Let's talk about this.
An OTT ultimatum is completely not necessary. Instead of throwing a hissy fit because he's doing something without her, they should talk about why this is upsetting her. As far as the whole mom moving back in thing, it's his house. If she has a problem with his mom living in his house because she may live there one day, she can suggest they move into a completely different place or request that his mom move out when she moves in. Until then, it's his house.