You are right in wanting to step in and remedy the situation, especially because your girlfriend is being physically abused. However, you really need to be sensitive to how your girlfriend wants you to handle this, if at all. I wouldn't do anything before talking to her first and seeing what she is comfortable with. She may want to handle this herself, or get an objective, third-party mediator involved. You don't want to force a confrontation between her and her parents before she is ready to deal with the situation and the consequences.
I think you should seek professional counseling, perhaps from a social worker or a school counselor. (Or if you go to a church, you could ask your pastor for some advice.) I always think it is better to be safe than sorry when situations get physical and that someone should step in and take action when appreciate, but you both want to be very careful that you handle it correctly so the situation doesn't get worse and blow up in her face.
In the meantime, you need to be extra sensitive and compassionate toward her while she is going through this difficult time.
Listen to Her
Listen to her, acknowledge her feelings, and know when to just hold her and love her, instead of trying to "fix the problem" for her. Confront her parents only when the time is right and she feels comfortable with the course of action.