In my opinion I would just cut contact with her for a while. If she's really unhappy with him and wants to be with you she'll realise it when she thinks she's going to lose you and reach out. If you keep contact with her she's in total control and can play you both however she wants.
That's just my opinion though, I've been in similar situations in past and have resorted to doing this, it doesn't always go the way you hope but why would you ever want to be with someone that doesn't want to be with you?