I'm a teenager (grade 12) but I don't think I can offer too much help. I know I could probably never go out with someone that comes over to my house alot. In fact I would barely want him to associate with my parents --- I am not sure why, it just makes me very uncomfortable. But since she's already been hanging out with him at the home...that probably isn't the problem.
I would place my bets on something that happened outside the home. Maybe he did something she doesn't agree with? I know that most things about me that my mother has to scratch her head at happens somewhere else. Infact everything LOL. At home with my friends I'm fairly good..but outside the house, I can be pretty wild, and no one in my family could ever guess it. Maybe at your house he acts like a nice guy, but somewhere else he's more comfortable and feels like he can be "himself" whomever that may be, and she doesn't like it.