How can i get a girl to like me when ses dating an ***?

the guy she i dating always calls her a lier and says that she is never doing wht she says and is real controling over her. she saysshe wants to break up with him and get her life together with me but she cant seem to break up with him an stay broken up with him. wht should i do to help her. cuz i feel that if i dont do anythingsoon i will loose her and she isthe most amazing woman i hve ever met and shes got them eyes that when she smiles she could light up a room when they twinckled.