I was seeing someone and we slept together and in bed he told me had a girlfriend and I freaked out and he told me it was a joke and he laughed it off. We can see each other’s locations but after we hooked up he distanced himself over a month and I noticed him spending the night at an apartment and not responding to me while there. Recently he seemed Interested in me again but I noticed he was no longer going to the apartment. I pushed away his advanced and sure enough the following weekend he was back at the apartment and ignoring my messages. I did some digging and the apartment he is at belongs to a mutual friend of mine and I believe it’s one of her roommates he is with. This man still flirts with me and tries to sleep with me and I saw he had an account on tinder. From a the friend of mine I believe he seeing a girl not sure if they are dating. Do I tell her? She is a mutual friend one could say as lots of my friends know her but I don’t want to get involved. Her roommate told me I should speak up and I’m pretty sure he slept with someone else as well. It is weighing heavy on my conscious that I may have slept with someone in a relationship and I was not aware and I can’t help but feel horrible for the girl. Should I tell her? If so anonymously or just personally with proof?