what to do?

don’t kill her yes you should be upset but maybe the guy was not really meant for you may be he was meant for her and don’t blame her alone cause if he didn’t want her he would not fell the urge to have sex with her the boy just don’t have any respect for you and your friend should know better but I advice you if she knows that you know about it then try not to bring it up around her she will feel guilty and like you don’t trust her anymore and remember he is now your ex not your boyfriend anymore don’t stop talking to your friend over a guy cause guys will come and go but a true friendship is just live loving a guy and you don’t really love that guy you only think you may love him but really and truly you just don’t want to see him with no other girl not that you don’t want him to be happy nut you might feel jealous it’s natural .all the best in findin love and with your friend