Dating someone new
And, chances are, he's not nearly as compatible with her as you were.
Sooner or later, your ex girlfriend will come to accept that this new rebound relationship just isn't working and won't be a long-term thing.
Few things are as painful and depressing as hearing that your ex has already moved on and found someone new.
This scenario is especially difficult if you're trying to repair your relationship and get back together with your ex.