Hey, karot. I think you should try to figure out why she's with a guy who doesn't treat her right. Then figure out why you're not treating your girlfriend right. You have a lot of stuff to think about before you risk your friendship with The Girl.

You're both young. Young relationships don't last forever. She and her bf will eventually break up. I think in the meantime, you should break up with your own gf, who also deserves to be with someone who loves her, and when your friend asks why, tell her you'd rather be alone that with someone you don't deeply care about. This will impress her, whether it shows or not.

This way you'll be available the next time she and her bf take a break.