I'm with Gordie. I felt sick when I read what you wrote about them doing things together. I have been there, I have been tempted like that, so I know exactly what is happening. Slippery Slope Part 1. That guy is not your friend.
I am not sure why everyone is saying not to bring your W into this. This man is a Christian friend so why don't you tell him very clearly that you are standing for your marriage and why, if you are, and ask if he wants to be a prayer partner with you as he stands for his marriage too. If this freaks him out and he doesn't want to, I agree with DnJ,you have your answer. End friendship with him and start praying the hedge of thorns around those two!
If you are not standing for your marriage, well, that's another story. But I would say that from the outside there was no question in my mind that those two should not be doing that unless they are inviting you and his W to come along too. Keep a clear head, no friend would do that, at least not more than once. Keep walking in the light! We are here with you!
I believe I will see the bounty of the Lord in the land of the living. Wait for the Lord with courage. Be stouthearted, and wait for the Lord.