Is it ok for your best friend to hangout with your ex?

This guy wasn't exactly my ex boyfriend but we hooked up and talked for a few months and I really thought he was going to ask me out. Instead he was a jerk and stopped talking to me. He really hurt me and my friend was there for me through the whole thing and has said I didn't deserve a guy like that. Now a year later she is hanging out with him and told me that I was overreacting when I got mad at her but why would she feel like she needed to text and hangout with him when she knew he was a jerk to me. I'm I overreacting or do I have the right to feel betrayed?