My FWB had a girlfriend

Ok so I knew he had a girlfriend but I thought me and him would never have sex but sure enough we did I thought it was only going to be that one time but as time went on and kept having sex and I feel so bad because I'm friends with his girlfriend what should I do ;^; should I end things with him or tell his girlfriend? I mean I really like my fwb which is a bad thing to do Dx