Hey there:-)
I think all you can do is simply tell her you disapprove, but trust her instincts. The more you try to push her away from him, the more rebellious she'll become - there might come a point where she only stays with him because either she just wants to spite you, or she doesn't want to hear "I told you so."
Be supportive, but let her know what you don't like. After all, she's an adult and on her own at college. She has to learn for herself. I truly hope she just doesn't get hurt in the process.
Good luck!