@Dontbeme
Get him out today, not Friday, not the weekend, not when your DH comes home, today. You need to show your DD that you mean business here and will not take any of her BS. You know she cooked up this plan with him to just land on your doorstep after being told no. So ring the police, tell them what's going on and have them remove him, they can put him in contact with any authority that can help him, as the dad has reported him missing the police can contact him too. Put your foot down OP, you know they are going to try to move him into your new home too.
This. You have to act now.
Yes, of course your DD is coming out with the “exact shit” you knew she would: she’s 15! She hasn’t got a fucking clue about life but she thinks she knows it all, which is entirely normal for her age. Your job is not to get sucked into arguing with her but to lay down the rules, and mean it.
Im very much for teenagers having autonomy where appropriate, but this is one situation where it’s absolutely not appropriate at all. It doesn’t matter how much she kicks off, you are her parent and you understand things that she doesn’t, and this is 100% your decision, not even a little bit hers. And you have to make her understand/accept that, one way or another, nothing else.
Call the police and SS, you absolutely have to get external agencies involved now. What is stopping you from doing that?
(And apart from anything else, as long as he’s staying with you he won’t be deemed to be homeless, so no chance of emergency council accommodation. You have to kick him out for them to recognise him as being in need.)