Maybe he is scared of being alone & homeless if he stands up to her? I can’t imagine having such a weak man as a partner or Dad.
Your DD is 19, by any normal standards, that’s still a young adult.
If you really think she wouldn’t be happy with you talking to him about this, then you are best not to - but only you know your DD, maybe she really wants you to, but feels she should say she doesn’t.
I’m not entirely sure what you could say that would make him behave any differently though. Not many people respond positively to being told they’re letting their child down and are being spineless wankers. You wouldn’t want to want to offer him a roof over his head if he stands up to his gf AND she chucks him out.
If he’s relying on his GF for a roof over his head, he doesn’t sound terribly solvent. Perhaps she booked a holiday for her, her Mum and her kids and told him he could come if he wanted to, it she’s wasn’t paying for his daughter to come too.
Perhaphs she’s been waiting for him to do some things around the house and he agreed to do them this weekend and when he made other plans (irrespective of what they were) she told him if he didn’t want to move out he needed to get them done THIS weekend.
Quite feasibly she’s every bit as fed up with him as you & DD. On the other hand she could just be a complete bitch. Who knows really?!
I’m really sorry your DD has had other issues and is vulnerable. And when she needs him her Dad is letting her down. It’s hard to stand by and not do something I know.
I think all you can do is continue to be there for her, remind her that this isn’t reflective on her, just him. Try to focus on how shit his behaviour is, rather than what a shit he is. Then hope the monumental twat wakes up to himself, but don’t hold your breath.