Or at least that's how it felt when I first heard the news, I'm calmer now 
They've been together for a year and GF's mum is one of those who think her daughter's her best friend. I have a good relationship with DS1 but recognise he has his own life and try to live by that.
We lost DH last year, so it's not always easy and TBH I was worried how we'd manage to celebrate DS's 21st birthday without it being a bit of a damp squib.
I needn't have worried as GF's mum has removed the problem by booking a week's holiday for them for his birthday present. An amazing present and DS is thrilled at the idea of a week away with GF, naturally he'd rather do that than have dinner with his mum, but it means I won't see my pfb on his 21st. I also feel a bit undermined as I won't be doing anything of that magnitude for him.
Anyway, I've told him not to worry, go and have a fantastic time and we'll do something when he gets back.
I'm putting together a birthday box for him to take with him. So far I have ready made cocktails, 21st birthday cupcakes, sparklers for the cakes, cocktail glasses and umberellas, a big birthday badge, party poppers. I don't want to make it stupidly OTT, but any suggestions for a nice finishing touch? He'll have his present when he gets home.