What if my girlfriend's parents abuse her?

My girlfriend cops a real hard time from her parents. I was aware that they use phisical punishment, but She's turning sixteen this year, which is why I thought it was unusual that her parents were still spanking her. Recently however I was informed on to what extream. Over the weekend, her mother thought that it was appropriate to knee my girlfriend in the side of her ribs several times for punishment.

I don't know what to do, her parents have made threats before to expand their punishments towards her, which would have to be extream. Their's no bruises on my girlfriend, but it's horrible the way they're treating their daughter. Not only that, because she's been brought up like that, she thinks that the behaviour is acceptable. I told her that it's not good that she gets treated like that, but I don't know what else to do. Although she really hates her parents, I still don't think she'd like the idea of taking legal action ect.

Get proof (picture, etc).
File a complaint with the police.

Given such proof, the authorities will investigate. If you KNOW she's being victimized, you're to blame if worse happens & you saw the marks, etc. Please get her help.