Unfortunately there is little you can do. Just accept the situation and make him feel welcome. After all if they're in your home you know where they are and probably what they're doing.

Believe me, if it comes down to choosing him or you she'll choose him. It's a well known fact that when a girl get's a boyfriend her brains dissappear out her ass and the same goes for boys getting girlfriends.

I had a friend whose 16 year old daughter chose her 42 year old divorced and unimployed slob over her parents.

Just make the best of it, it's all you can do.