I feel betrayed by a mutual friend :[

"I hope this isn't a silly thing to say but tonight I am annoyed at mutual friend for inviting an ex-friend to a server we're in without letting me know in advance. They're aware of our conflict, it's not like they don't know but I feel like if you know two of your friends are in conflict with one another, you should let one know when the other that they're at conflict with will be in the same space.

I believe I'm mostly frustrated because that was one of the few servers I felt safe to 'cool down' or 'relax' in since ex-friend wasn't there but now that he is, I feel uncomfortable once more. I don't want to be rude to our mutual friend, but I'm upset he didn't warn me in advance."