My ex cheated on her ex with me (I didn't know) and then discarded me; I later found out she likely cheated on me with the person she's now engaged to. Do I try to tell him?

The title pretty much sums it up. My ex girlfriend and I were together for about half a year, attached at the hip. Things went off the rails suddenly once it became a real relationship, and I discovered she had secretly been with her ex-bf for about 40% of our relationship (we were LDR).
I recently found out through some people I met up with that she's engaged and she met the guy...the same month we were together/broken up. He has no idea. Some of her friends know me, but if they see the overlap, they haven't spoken up to him about it. And her public story is basically I don't exist.

She's been flat our narc post breakup. Tried to cause drama, cyber stalking, the works. So it would be risky for me to do it. But she destroyed two men in less than a year already, and is now teeing up a third guy for another round. Part of me is concerned I'd be kicking the hornet's nest; another part of me wants to spare a man the potential suffering she's caused me and her other other ex. Thoughts?