Tell him?

Sooo long story short me and my ex of over four years are trying to work things out. While we weren't together he knows i was with other people. However, the one guy I'm still friends with. He stopped by my work today and we had a chat in his car about normal stuff. We were friends before anything ever happened and tbh i don't even remember the night and if i could do over, it wouldn't of happened cause I'm not even attracted to him. He did used to flirt with me however i think hes realized there will never be an "us". Should i tell him about this or? Not saying what's right and wrong here. Everyone is different.