I have mixed feelings about a situation happened to me.
I went out with this girl and didn't work out but she remained in my group of friends.
Few days ago i went to a 30ies party i organized for a friend of mine. There that friend had sex with the girl i dated and it wasn't possible to not hear them from where they were-we were 10 people and sleeping all at hearing distance even if divided by thin walls-this friend was the friend to whom i cried about my lingering feelings for her till few weeks before. And even her probably knew of that even if i was trying to accept the rejection because seemed definitive. Both of them had weeks to let me know they were dating, or could at least went outside in one of the cars to do that. Can i feel betrayed by him? What would you do in my place? It's also the friend that in the past months i helped to get over his break up and reached out over the years because he was feeling unwell.