I think Chris, this is far more common than you might realize. I recall a similar feeling...Oh I ended it alright - long ago and far away...Then? DID marry "the love of my life". I was restless later on...Annoyed, actually. This guy professed to love me and I spent my youth in a relationship with him, the only one ever. Turned out? He screwed around behind my back with EX-friend(S) He thought I didn't know or wouldn't find out. When it blew up? I had my say and walked away. For whatever crazy reason? Almost, 20 years later, he was at a reunion and I contacted him hoping I would see him there (stupid, I should have just gone and taken my chances) He thought it was for a rendezvous and when I realized that? Wow. I realized how very, very stupid I was because what I REALLY WANTED was to rub in his face I'd married the REAL LOVE of my life; had a family; a successful career and looked like a "million dollar babe". Oh the "webs we weave". So I played it that way and simply said, "I actually just wanted you to see my life turned out in SPITE of the great betrayal and wrong you did me all those years ago..." I smiled he looked aghast and mumbled "sorry" and walked away. The point is?
I carried this "unrequited" desire to SHOW and TELL to spite him and actually, it did work. My husband and I later had a discussion about it, since there was a "girl" in his past he had an encounter with (the great part about that? She was 150 pounds heavier when he saw her and ANY feeling of the past, if there was any, was utterly dismissed...I think he was in shock!)
Your wife may have some unresolved issue with something he said OR what he did during their relationship OR how it broke apart...This Chris, is very often what counseling serves to do. I wish I HAD thought about when I did get some marriage counseling, but it wasn't about anything like this issue of longstanding.
Maybe she would benefit by going to a counselor and spilling this and maybe that person could help her to DETERMINE just WHAT it is that is DRIVING this desire...It could be? Nothing more than life responsibilities closing in and a desire to return to the past. Or it could be she wants to SEE him to ACTUALLY SEE how he has turned out in spite of all...I really don't know.
Some people would say? It's dangerous to venture forth but I can tell you from both my husband's and my own experience with these long ago "loves" ??? -That by seeing them again after 20 years, it was totally dispelled...Washed away. Now I realize? That DOESN'T happen with everyone and I think? That is your GREATEST FEAR.
I hope I haven't upset you, I just wanted you to realize it happens; it can be hurtful; it does need to be examined and maybe it's not so much the past? But a desire to renew a more romantic relationship and do the little things for one another that you both use to do in the beginning...For sometimes? WE FORGET. Life get's FAR TOO BUSY. Which is why I always advise so many here to get away for a few days, just the two of them, do something fun and TALK. That's one way of clearing the air and coping.
Were I your wife? I would now seek some counseling about this - some direction from a 3rd party...It couldn't hurt and might shed more light on all...
Best of luck...
Grace