First, dont jump to conclusions. The girl is his ex and might not be over him. Or they may not "be over" but unless he has told you specifically that they are or has beguna a boyfriend/girlfriend relationship he is doing nothing wrong.
Second, dont continue contact with this girl. Dont take her calls, dont answer her questions, dont let her bother you. Females are devious, you dont know her, dont trust her. Think about it, what good could she possably bring to the situation.
Lastly, just talk to the guy about it. Dont over react just have a simple smooth conversation about it and take it from there.
Good Luck Hun!
* ~ JeN JeN ~ *