Your situation sounds quite familiar.
Right after DD1, 6 years ago, now, I said I was going to call OMM and tell him to stay away from my W. Man, talk about an explosion of ape doo-doo. She threatened to divorce me immediately if I did. I got several versions of the, "He is innocent" speech, his wife would be hurt unnecessarily, and I was just being vindictive. It was very angry and emotional on her part.
She also swore she was not seeing him any more and never would again, so there was no reason to contact him.
So I didn't.
After DD2 I had no qualms about confronting him. I mainly wanted to determine his intentions. It was not pissing on my territory, as W accused. And I had no plans to confront his W. After talking with him I decided I wanted to keep it as leverage, in fact.
I learned from him that my W had not even told him I knew way back then. She lied to him, her soul mate, and the one true love of her life, as much as to me. I will never understand that. They were so connected, right!
But I do understand she did not want me to talk to him back then because she had no intention of ending it. She said similar things after DDay 2. This time she added she just wanted the damage to stop here. That irked me no end. She expected me to shoulder the whole burden. And she wanted to protect him. Sorry, no way did he get off scott free this time. No way.
He was pretty angry with my W when I talked to him. He blamed everything on her. He was upset no end that she lied to him, strung him along, and was such an immoral person. I could go on. He wiggled and squirmed to avoid responsibility. I was gagging through most of it.
I never did expose to his wife. She found out on her own. I talked to her then. But that's a whole 'nother story.
W did not agree to NC letter until after his W found out and OMM called me and again blamed it all on my W. Even threatened a harassment suit at work. Finally, W saw his true colors shining through. I date the start of our recovery from then.
Moral: WW not wanting you to contact OM is a big red flag! It's all the more reason to do it.
T