Ask her how many times he's said sorry for his behaviour...if its more than once and he continues to do it, she has a seroius problem.
If it is more than once (and it sounds like it is) explain to her using an illustration. Say "you've seen him 3 times, he says he loves you and he keeps hurting you and saying sorry, and you keep taking him back. What if a friend kept hurting you, would you keep them as a friend? This guy is bad news and I don't want to see you hurt. You need to get rid of him regardless of what he says, he's not worth it."
She'll get over it, you sound really young.

There will be lots of guys coming and going.