How do you convince a girl to break up with her abusive boyfriend?

This girl I know has an abusive boyfriend and it really concerns me.

Shes a freshman in high school and her boyfriend is 17. I have no issue with that but its the fact that hes extremely abusive and makes her feel like a piece of ****.

Every day i have to hear her tell me about how he either hit her or said a ton of mean things to her like shes a worthless piece of **** or shes only good for sex.

How do I convince her to break up with him?

Ive told her before she needs to and she says she knows but she can't stand to be alone and shed probably just kill herself or she tells me that shes too attached to him.

I know other guys would like her because shes actually really pretty. Please help me out.