Should I tell her?

So I was seeing guy for almost a year and about 4 or 5 months ago he started seeing another woman as well as me. I just found out about her last week because his friend unknowingly told me about her. Turns out I was a secret the whole time we were together and his friends didn’t know about me. The friend that told me about the other woman figured out that we were an item and is the only who knows the situation. I obviously broke everything off. Now I’m on the fence if I should tell the other woman.