Honestly, I'd be willing to give the guy the benefit of the doubt based on what information you have provided here. It sounds like he was involved with her before he found a more suitable partner in you and he's aware of the emotional damage he just committed. This may show that he is actually a compassionate man that cares about how his actions impact others. He may believe that explaining in person would help her cope, even though it may not and she may only need time to move on.

I think you should just keep your eye on the situation. Your BF may simply need time to break the emotional connection that was there. If he doesn't cut ties with the woman after another month or so, then worry.

IMHO