Should i tell him

A few weeks ago me and my boyfriend broke up. I had no intentions of getting back together because I was tired of being lied to. But while we were broken up I had hung out with a a friend and we hooked up in his car. Me and my boyfriend are both friends with him. I’ve known him for six years and my my boyfriend known him for about 5ish years. Should I tell him about me hooking up with him when we were broken up? My one friend thinks I shouldn’t since we were broken up but i don’t know what i should do.