I wouldnt even bother with her. Do you realy want to settle for second best being with a girl who clearly loves someone else?she sounds to me like she wants to have her cake and eat it. If you let her walk all over you this time then shes going to know she can get away with it and do worse things.
Do you love her? because i really think if you did you wouldnt be able to share her with anyone else?
Maybe talk to her, ask her what she wants straight up, make her choose between you and him and if she chooses him then she wasnt worth having in the first place. if she chooses you then you know its you she wants to be with.
Good luck