First of all, don't panic. That's going to make everything else worse.
There will be those who say expose (i.e. tell everyone) and others who say this is punitive and doesn't help things and instead pushes the WAS away.
I did a sort of exposure, which backfired on me because the society I live in doesn't seem to condemn adultery anymore. People didn't want to take sides - they just wanted to plaster happy smiles and go along like they always did.
My personal advice is not to do anything that will make things worse. She's blaming you for what's happening with OM. If you expose, will she become angrier and vilify you more? Those who support affair exposure will say that this is a temporary state of affairs and she's more likely to end the affair.
I have tried to break up WH and his OW a few times. Each time they wobble but they never break up. It's like the night of the living dead. I think, in fact, in the early days when I was really fighting for him, the OW loved the drama and the excitment and the more I fought for him, the more WH had value in her eyes. I feel, in my case, if I had gone, 'Meh - you can have the lying cheating scum', she might have not been so keen.
I am still tempted to try to do things to break them up. But in the end I feel the WAS should come back willingly. Otherwise you will always be looking at them out of the corner of your eye, wondering if they're really 'with' you or faking it.
I would say something like 'OK, if you want to go, you're free to go. Sorry you're having problems with OM (while mentally thinking: if you didn't want such drama then don't have an affair). Wish you all the best.'
