I have a simple question, just for clarity.
Thus far, from what you have told us, your assumption that she is still "with" this other fellow is based on what you overheard after her phone conversation?
I'm proposing an alterate explanation, before you go and get all riled up.
Could it be that, when asked if that was him on the phone, if she just didn't feel likely explaining that "yes it was the same guy but they were no longer together blah blah blah etc etc". Maybe she just fell like getting into it and so just said "yes".
I don't pretend to understand women, but one thing I HAVE learned is that soemtimes if something is kinda complicated they just don't wanna talk about it.