Going out with my baby daddy's best friend?

My baby daddy and me was together for nearly 6 years but he left me and eventually moved on and is married now. His best friend since 1st grade messaged me and started hitting on me and asked me out. I tried to tell my ex but he didn't believe his friend would do that to him. I really wanted to make him jealous and mad so I agreed to go out with him. The next time I showed him the messages and he believed me so he got mad at his friend and went to his house to confront him but he wasn't home. So my ex made me block him and not talk to him. Then me and my ex had a big fight and we don't talk now at all and probably won't ever again so now I'm talking to his best friend again but he doesn't know. Should I tell him? 

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