That is great that your friend hasn't physically acted on it yet.
There are 3 relationships that he has to deal with
1) him and the wife: that screams couples therapy
Anyone that is in a stable/loving/monogamous relationship will meet people that they find attractive and want to bang unless they live under a rock.
Doing that is another thing. Due to the commitment that you have to your partner you are polite and don't act towards ****ing the 3rd part.
Your friend has gone beyond that point and is acting towards being with the other woman that is pretty much cheating not in a physical manner but emotionally for sure!
I guess that he should first figure out what is going on in his head, heart and underwear.
Is he bored? not attracted to the wife? have they turned into roommates? Is he crazy? Just horny? the list goes on and he needs to dig deeper to know the answers.
You are his friend but not a professional tel him to talk to someone that know what he is doing.
2) him and the other girl
He is married, she is not!
The ethical thing to do is: put her on hold while he deals with the first issue.
If those feelings are real and strong enough everything will be fine at the end.
3?) him self
The dude needs to figure out what he wants and act towards it!
What is more important and what makes him happy.
One of my best friends is divorced 2X just because he wasn't happy and that for me is the best option.
Maybe your friend love the wife for various reasons and they are great friends but not a real couple (here at home there is a rule: the day you don't want em anymore please dump me).
Life is made of priorities your buddy has to decide what his are.
PS. A lot of people here seem to be more interested in sharing their judgment than helping... interesting
