Here's the thing about exposure: NEVER GIVE YOUR SPOUSE OR THE OTHER WOMAN/MAN WARNING THAT YOU ARE ABOUT TO EXPOSE. JUST DO IT!!!
Why?
Because that will give her and the OM time to get their stories straight/corroborate timelines and make YOU out to be the crazy/psycho husband who has trust issues and is going through a hard time in his marriage, therefore he suspects his wife is cheating on him and wants to lash out at everyone. They WILL do this if you keep giving them warnings. Oh and you bet she's told him already "My husband knows...if someone asks we can just say we're friends" and have already started planning and concocting their stupid excuses and lame cover up stories.
Find out who his wife is and exposes immediately:
"OM's Wife,
Your husband, Name, has been having an affair with my Wife's Name since on or about Month/Year. I discovered the affair by way of (fill in the blank). (Copy/paste or verbalize any proof you have). Their affair has been detrimental to my marriage. My wife told me the affair ended however I have proof contradicts that--they are still having an affair and in contact. I am telling you this because you deserve to know the truth. If you were already aware of the affair, then I am sure that this comes as no surprise to you, but if not, I am sorry to have to be the one to inform you. If you want to talk further or need further proof, you may contact me (at....fill in the blank).
Your Name"
THIS IS HOW YOU DO IT!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
F them! And no, your wife doesn't sound like he wants to work it out with you if she's still lying to you. So expose her for the liar she is without telling her or OM. And in the interim, tell her "I am aware you are lying to me and still in contact with the OM. You need to leave today because I refuse to live in an open marriage. I refuse to be treated so callously and be lied to and I will not tolerate this nonsense and your betrayals anymore. Get the f*ck out, homes!"
Remove yourself as an option for her. She will not feel any consequences as long as you're covering up the affair and allowing him to stay in the house/carry on as a married man with all the benefits of a committed relationship. She is not committed to your marriage, therefore you do not need to reward her with the same generosity and pat her on the back and feel fearful when she is the one making these d!ck moves. Tell her where to go.
Oh and a word of advice: NO marriage counselling as long as she's having an affair. MC does nothing as long as one partner is still lying/having an affair.