I love a girl who has a boyfriend, can anyone offer advice?

Well she must be intimidated by the thought of leaving him especially if he his abusive (physically/emotionally/verbally). If you know for sure that she doesnt love him, than just express your feelings to her and ure her that a life with you is better than staying with him. It's really up to her to make the decision, so all you can do is tell her how you feel and let her decide.