OBVIOUSLY, keep on hanging around with her, dating her, while she is still in contact with her Ex. That way you show her who is more loyal to who.
...yeah, I am kidding. Dude, you said your piece (or is it peace?). If she is still in contact and it is less of a "they-mutually-seperated-and-she-assured-she-wants-you" and more "they-talk-for-HOURS-and-I-get-maybe-ten-minutes-if-I-am-lucky"...tell her it is not working out, and like all who say, move on.
Because it will be a depressing and hurtful road if you continue walking down it.