Not accepting between him and his female co worker

Hey guys, I’m trying to forget about the past messages between my fiancé and female co worker aka friend. I do understand of working together but why do you have to communicate with her outside of work? I’m not cool with them together. Just her newlywed husband is okay with it but not to me. I will never forget when she told him I’m single and she show him some pics wants to be a model. Now I’m engaged because she is married? It’s look strange to me he doesn’t want to stop being her friend. I want to accept it but I can’t I don’t trust her. It’s either her or me. I wish he could see that and he knows it. I don’t know why he’s won’t let me go.