Sorry to be later coming back to this than I'd planned.
Having reread your posts, it seems you are saying that it's because of her retaliatory remarks that this has now become so pressing an issue? Perhaps also, because others know about it now - and she has said that you are the most censorious, this has made you feel quite lonely in your views? That's really quite unfair of her, you know.
When someone is doing something they know to be wrong, they simply have to find a justification for it - and they also tend to hit out at anyone who isn't buying the justifications, because that forces them out of their denial. I suspect that this is what is happening with your friend.
Realistically, no-one can square deceiving someone else for a long time; there is simply no justification for it. So this is where the bargains and the denial come in. Her response is to counter-attack with accusing you of simplicity. This is all part of an internal script she is following though. She thinks this relationship is unique in its challenges, that it's somehow different to every other relationship with a MM. I'll bet she's coming out with gems such as: "This isn't all back and white, there are shades of grey that you can't possibly understand..." and "You don't understand him, he's a wonderful person...he's not like any other MM having an affair."
Meanwhile, OW the world over are coming out with the same old lines, believing that their circumstances are unique and that others simply don't understand .
Reject this accusation of over-simplifying then, OP because actually, what's needed here are some unassailable truths. Neither her not her MM are intrinsically bad people, because good people have affairs, but there is no getting away from the fact that deceit and lies are bad - it's their behaviour that is bad, not them as individuals.
My friend story is rather similar to yours. This friend has been close to me for over 30 years. She has a history of chaotic personal relationships, but she is a good person and I love her dearly. She actually pursued her MM, whom she met at work. It seems he wasn't really looking for an affair, but having been married for 20+ years and with grown-up DCs, it seems he finally said "why not?".
He told my friend initially that he wasn't unhappy in his marriage, but wasn't deliriously happy either - a state of affairs in many long marriages, really.
Once the affair had started in earnest though, he followed the same script as many adulterers - finding fault with his wife and complaining to my friend that his wife didn't value him enough; in effect, finding himself a justification for what he was doing. Withdrawing from his wife and blaming her response behaviour.
Not that my friend saw it like this at all - she had bought all the tales about the unaffectionate wife who was more concerned with her high-flying career than her H.
That is, until the "uncaring wife" treated the H to a wonderful present that he had wanted all his life - and it turned out she had put months of effort saving for it and choosing it, which blatantly contradicted his story about her not caring enough .
The main reason he gave for staying with his wife was that he was waiting till the youngest went to university.....but you guessed it, he was still there a year afterwards.
Now fortunately, her and I have always had the type of relationship where we can tell eachother how it is and because I know her so well, I knew I could be quite challenging with her and knew what buttons to press in her basic character and personality to get her to think this through more rationally.
I told her at the start that I couldn't condone this relationship and that if she wanted to talk to me about it, I would be very challenging with her. That left it in her court whether she chose to speak to me about it. Fortunately, she decided to do just that. I then did what you are doing, kept finding holes in his story, to the extent that she told me that although he'd never met me, he wished she'd stop talking to me about it!
I got her to read the book that I always recommend on here "Not Just Friends" and managed to persuade her to go to counselling. In fact, her counsellor wasn't very challenging and seemed intimidated by an older feisty client like my friend, so she stopped that and found another who understood infidelity, from all sides of the triangle. This counsellor was far more interventionist and challenging and my friend often said it was like the counsellor and I were comparing notes and acting as a team.
Both the counsellor and I got my friend to look at all the stories in the triangle. It won't surprise you to read that the wife's story was being woefully ignored by these two. As I said, my friend is at heart a good person and a feminist too, so I knew that although she really didn't want to, once she forced herself to empathise with the wife's position, we would make headway. This was the first real erosion of her denial.
I also helped her to project the various outcomes and what that might look like for her. We had a lightbulb moment when my friend worked out that even if this man left his wife, there would be times in their relationship (my friend has got very challenging older DCs herself) when his ego needs would be at the bottom of the pile, because that is life at times. What would he do then if yet another woman pursued him? Would he be saying similar things to a new woman - that he wasn't unhappy, but that he didn't come first at the moment?
By this time my friend had come a long way. She started by thinking that she would be able to prevent his future infidelity by being the ever-loving and attentive wife, to realising that this hadn't worked in his current marriage, so why would she be any different? She actually started to see his selfishness and how this was all about him. There were other clues of course; his self-absorption with his own problems, his lack of care and understanding of her DCs' problems. But by now she was noticing these things and not bargaining them away, like before.
I took her for a weekend away last summer and by the end of it, she had the courage to end this relationship. She looks back now and can see things so much more clearly and has a lot of guilt about the damage she caused to another woman, but her fantastic counsellor is helping with this, as I continue to do.
The man? Still with his wife and enjoying child-free time with her now that the DCs are away from home....
This is a long story, but I hope it might help you. Your friendship might be different to the one I have with my friend and only you can know whether yours can withstand this level of intervention. My friend always knew that I was doing this at her request and because of the deep love I have for her. She knew I wanted only the best for her.
I understand what you mean when you say that this man is such a big part of her life, it would be difficult not to talk about him - and I think had my friend not chosen to talk to me, it would have created a distance in our relationship. But that's a risk you might have to take, if you are going to stay true to your morals and your beliefs, which you should never feel bullied into compromising.
It helped in my case that our other friends were on the same page as me, although once her and I started talking properly about this, we had an agreement that I would not discuss this with the others and I stuck to that confidentiality throughout. However, she told me that she would often go to one of the others and tell them what I'd said - and fortunately, they backed me up.