My WW's mother knew she had kissed another man and then when she finally admitted to the affair post DDay her mother was almost proud of how long she was able to hide it. My WW also confided in 2 friends, no one said anything to me.
Here's the issue that I have, before I was involved in this situation, I probably would have said that a friend is often a confidant moreso than the morality police. One of my wife's friends told her that it was a mistake (she didn't let me know) but she told her it was wrong. Before I became a BS, or rather, before I was aware I was a BS, I probably wouldn't have said anything either, in fact, when my "friends" were doing stupid things like this, I didn't take an active role (ironically, I knew my WW's AP was cheating on his GF, he was a friend of mine, I did tell his GF and she was really not interested). So I don't hold a lot of anger towards her friends, I don't think they know what all this is like and they didn't encourage it. I can understand that they may not have wanted to interfere, even though I would have liked to know. I can understand their inaction.
My MIL on the otherhand is a WS in her own right. I limit my interaction because She should have reacted completely differently. I don't tell my MIL I love her, I don't spend time voluntarily with her, and I avoid her at all cost. She's my MIL so I don't want to cut her out of my life, nor the life of my child.
I do agree with BFTG though, these people were enemies of your marriage, in the end no matter how much logic I can put on it, they helped my wife hiding it because they didn't out her. I can understand why people completely cut them out of their lives.
I think that had they supported the affair by means of encouraging it and helping the lies, aside from just omitting them, I would have completely cut them out. The people you describe cannot be beneficial to reconciling.
Me: BS Her: WW - Sayuwontletgo
Married 14 Years, 3 Kids
DDay: Oct. 14, 2017
3yr LTA, Found out years later
AP was a friend