So I have an update. I was initially surprised but the more I thought about it, the less I surprised I'm becoming.
On my friend's side, she said that they have been working on their marriage . He took her on a trip to reconnect and they're doing fine. He keeps giving her updates about the "OW", which I found weird, but I was going to leave it at that. However, this morning during our team meeting, our Deputy Director brought up his name. Apparently, there was a follow-up trip to the previous work trip in the OP. She said that the team that went to X, went back to finalize whatever and that we need to liaise with them. Them being him and the other woman only as they were the only ones who went back. Now, my friend did mention to me that her husband went on a work trip recently and after she picked him up, they went out to breakfast etc. We speak nearly daily so it was part of our daily conversation. I did find it odd that he would've done 2 trips to the same place so soon (there was a conference then two weeks after this) but didn't think about it deeply or anything.
I do not know if anything happened. I am at a point in my life where I value our friendship but do not know if I should bring this up. This would change our relationship for sure, and I do not know if anything happened, however, I do know that my friend does not know that she went. I don't know what is the ethical decision to make here and I don't want to lose my friend over this.