We had a joint friend that, while he did not know of my FWHs adultery, did hid the fact that the two of them had had a guy's night out (this part with my blessing) that included lap-dancing and paid grinding against them (not with my blessing) by strippers. They conspired to NOT tell me about it. And this is a man that I considered to be equally friends with me and my FWH. All this came out in the timeline that I demanded and was given.
We were invited to go to this friend's major advancement in rank ceremony on the east coast, a trip that would include a cross-country trip in the winter, staying at hotels, meeting with him and his new wife, and meeting up with the families.
I emailed him, a rather long email, and called him out on his betrayal of me. I told him that I was uncertain if I could attend his ceremony knowing that he had conspired with my FWH to keep such a betrayal from me. And to his credit, he responded by apologizing sincerely for conspiring, for letting himself get drunk enough to think that this was a good idea, to let me know that something like this would never happen again, and to tell me that he would fully understand if I could not come to support him. My FWH and I did fly out and have a lovely time. Had he not apologized in the manner that he had, this friendship would likely have been terminated.
So, what do YOU need from this situation? An apology that you might or might not get? NC from this point on with the "friend?" What? Do you see yourself, under any circumstance, being able to forgive this "friend?" Do you even want to go there?
Dependent upon what you want, you may just want to cut this person from your life from this point on. Personally, I just don't think that I could ever forgive anyone who helped my FWH with his A. It was hard enough to get to the point that I could forgive him, and I love him. I don't think that I could ever love a third party that much.