In my view As require 3 things. 1. Opportunity 2. A Character deficit 3. Privacy/Secrecy.
Your W has free agency as an adult human being. She told herself something that allowed her to make a choice that was against her morals and character. This is what she needs to look at and work.
OM no doubt played a part. He provided the opportunity and some of the secrecy. She could have said no. She could have told you all about him. She could have told his wife.
How can you feel safe is she is this susceptible to a "maniipulator?" Really think about that. Why would you want to continue in a M where she is at the mercy of the next narcissistic smooth talker to come along.
Of course she learned her lesson and will never do it again. I call bullshit on that. It is almost always the first thing a WS says. Or that is was so unlike them. . .
How many WS got M'd thinking they would unfaithful one day ? I bet almost no expected to be a cheater. Because that is not like them.
Here is the kicker. It was them and they did something that they never expected to do.
OM is not your problem. Your W is. I would expose this asshole to your social circle before another one in your circle falls victim to his charms. Real friends will stick by you. Fair weather friends will not. Those people were never really friends to begin with.
I know the inclination is to find anything beside that our spouses choose to have an A is strong. We are trying to preserve our image of them because we still love them despite them hurting us so badly.
Your W needs to get into come IC and figure out how she was able to convince herself why she was so susceptible to his charms. Again, how can you feel safe in your M and build another one without her making changes. If she doesn't you may end up in the multiple A club.
I know I sound harsh and I do not intend it to be. I am happily R'd myself going on 7 years post dday. Right not your are buying into her justification's for doing this. The real reason she choose this remains buried underneath all of the things she told herself to make "ok" in her mind. She needs to dig deep and figure that out if you want to stay M'd to her. If she can't you have to determine you are willing to take a risk without her making changes. The risk will remain.
You can't rebuild a M in the exact same way the previous one was built. It robs your of the life, M and W that you deserve.
What is she willing to do to make changes and figure out why she gave herself permission to do this. It might be buried deep in her childhood or adolescence. I guarantee you it is there. This is not his first "attempt" at this. He has failed in the past, but why did he fail with another woman, but succeeded with your W. What is different ? There was a time where she was likely intoxicated by the "fake" admiration he gave her. Or she felt she would never get caught so felt she "deserved this."
You need to be very firm on your boundaries right now. Those include buying into her justifications (call them excuses if you want). I don't think IC is bad idea for you either. It can really help you organize your thoughts and see a way out of what your are currently living in.
That might mean you stay M'd and if you do, your M needs to be different. Not worse, just different. Or this means that your W choices are a deal breaker and you move on taking your kids best interests to heart.
I am sorry for being direct, but at the beginning the choices you make now set the tone for how your R, D or rugswept M looks like from here on out. You have to delay the pain sometimes and let your rational and logical self call the shots.
I am sorry you are here. Welcome to the last club you ever wanted to join. Keep reading and posting. There is a lot of wisdom here. It is up to you want you take in and what you ignore.