He's only 20--not even legal to drink. Why do you allow him to do it in your home? It's YOUR home!! I don't care who buys it for him. If you don't like it, don't allow it in your home.
The only thing you could do regarding the girlfriend is to tell her you won't allow her to bring alcohol she's purchased into your home. And you should tell her that very clearly to her face, then stay strong.