He needs to STFU, shove his feelings up his and move on. If its too difficult for him to stay with the ATF arrangement, he should give it away. Not worth anyone's pain.
Def not say anything .. It can make a situation very sticky unless she has expressed to him directly some sort of feelings for him..
Just my opinion that it can ruin a good thing and friendship.
xoxo
Secret Encounters
Let your friend make his own choice...as long as he understands the risks, and willing to accept responsibility...for his choice.
Your obligation, as his friend, is to be there for him...to congratulate him at his wedding, or pick him up and help dust him off...after he falls flat on the ground.
Either way, I wish him well. Life is short...no one ever gets anywhere without taking risks...or making a few mistakes in our lives!
As wise SA provider told me the other day, what ever the relationship is, just enjoy and savor every moment with her. Don't put any parameters on where it can go or can't go. Just be sweet to her and help her be the best person she can be. Worst case scenario is that you part with wonderful memories and her being thankful to have met you and a better life for her.
Danger ahead ....
I fall for my ATF every time we get together. ..thats good enough for me