especially since he had some sort of infatuation with this girl since the day I've known him
Wow. That really does make it unacceptable. He's been infatuated with her, before you two were an item?
In that case it looks like he wants things to develop with her.
The FB searching is more than curiosity IMO.
He's crossing boundaries and he knows it. It's inappropriate. So manywomen who became the OW have said this is how it started.
He gets me. He's so understanding. My soul mate. We were just friends. He listens to me...He's so patient Didn't mean to fall for him...etc
You can stand by and let the cards fall where they will... keeping an eye on his phone.
Or you can let him know your opinion on maintaining boundaries with the opposite sex.
A good illustration of that is the book 'not just friends' by Dr. Shirley Glass. It's available online.
You can't make someone stop what they do...but you can decide it's not a relationship you'll stay in under those circumstances.
I explained something similar to my DH some years ago. He was in contact with an Ex and some conversations were inappropriate for him to have with her.
I told him to cut contact. He didn't want to ... I said fine..in which case my plan is to emotionally detach from you I'll get in contact and maintain friendships with my Ex BFs if he thought it was ok and I'll leave when the DC are older.
He tried to say it wasn't the same if I was seeking them out..whereas he had always been in contact with this Ex.
He didn't like, but i told him I would do it and I was serious.
He insisted there was nothing wrong with it until I said let's ask a bunch of random strangers online.
I found a forum...got him to write it out.. every response agreed with me..both men and women. Told him he was out of order and his wife was right to not like it. It was a no brainer.
He was shocked at how no-one supported him and eventually agreed it wasn't good and he could see my point.