Most Helpful Girl
If she wants to stay in that relationship, it's up to her. You can express your worries or go after her, but I doubt she'll listen to you. People who are in abusive relationships usually don't see it, or don't think it's that bad. Maybe talking to one of her friends about it (the nature of her relationship, not the fact that you want her for yourself) could be useful.1
Most Helpful Guy
If you care about her, protect her, be her best friend, and get her out of this relationship she is in and give her a new and better relationship, with you! Does she feel the same towards you? Does she even want to get out of this relationship, does she want to be saved?
Talk with her about it and then do what you can to help her. No girl should be in any abusive relationship.1