How strong do you feel about letting her find out?
It can be quite a risk. The worst thing that can happen, is detailed in newspapers and the evening news on television, about love triangles gone wrong and someone gets killed.
Is it possible that either of them could extract enough revenge to do some type of harm to you or, possibly enact some type of retribution to cause you embarrassment, or possibly loss of your job, and your reputation?
Those are a couple of questions that you need to ask yourself before you do anything. You know he is already a little unstable. You are dealing with a liar and a cheat. She could be even worse.
You might be better off, just moving on and trying to forget.
We only miss what could have been. I know I don't miss what it really was.
I'm the OW, too, but I would not tell her just to hurt her....my partner is going thru a divorce anyway; she'll find out soon enough.