It sounds like you didn't have much going on with this guy to begin with.

I don't think its horrible that your friend wound up dating him. I don't think she is a bad person. People move on. This guy moved on from you.

I had women accuse me of dating their ex-boyfriend saying "friends don't do that". Problem is, these women were never good friends to me. So I did not consider them my friend to begin with. I suppose if they'd made more of an effort to be a loyal friend to me, I wouldn't have dated their ex boyfriend.