Should I tell her that he’s cheating anonymously?

I recently found out (after a YEAR of dating) that my boyfriend has had a girlfriend the entire time we’ve been together. I broke up with him but should I tell her anonymously about his disgusting ways or should I walk away from it?

Part of me wants to tell her because if I was her I’d want to know

But on the other hand, I wouldn’t want to cause unnecessary drama in her or my life.