Hello, Jim, welcome!
(And sorry for the late reply...)
I'm sorry you're both in such a difficult situation. It's clear you're committed to your relationship and want to work on overcoming the current issue. That's great, that's the most important thing for success. Yes, you'll need also a good counsellor, but if the one you see doesn't help much, you can always try another.
What your wife experiences is very natural and quite common, in various situations. I think the key is to understand it much better (although you both seem to have quite an insight already - that it's transference due to the similarity of their situations, that she feels as she does about him because he "gets her" (in this one regard!) so much better than others). Feelings of love / infatuation are very strong and even "blinding", but they pass. She (nor you) cannot try to force herself to stop loving that person - it wouldn't help, that's impossible. But she can (in couples therapy / counselling) learn more about her feelings and the two relationships (with you and with him) and then accept the very different roles the two men have in her life and stay with the one who she wants to be married to and raise her child with (i.e., you). She can decide when the times is right, to stop contact with the other one and let her heart heal from the amorous feelings for him. They won't stay; they never do (mainly if not 'sustained' by something). She can still hold him in her heart as a person who once was very important, who helped her and gave her also some nice memories.
As I said, it's important for her to make the decision when the time is right - to "break up" with him, to let it all go, to wait until the infatuation fades away. I think if she felt forced to do it, she would still keep a deep regret that would be painful and might perhaps even cause some resentment towards you. That doesn't mean you shouldn't let her know how important it is for you (that she'll be able to resolve this issue and "be back there (only) for you"). Just also let her know you understand she needs time and help and that you're also willing to work on it with her, in counselling.
As one of the most famous couples therapists, specialized on cheating, Esther Perel, says, cheating often makes marriages stronger, if the couple works on the underlying issues. Do you know her? I know your wife isn't cheating, but some consider this kind of relationship (she has with him) "emotional cheating"... so it can be helpful to look into this topic. (I think it's a big advantage for both of you that she didn't "find someone else" because you weren't "good enough anymore", but just because of a very specific circumstance. But the emotions around it can be probably similarly difficult to 'bear' and to 'overcome' as if it was cheating. And even though "you weren't the cause", there can certainly be place for improvement of your marriage during this healing process.)
You can start, for instance, here:
Another excerpt, potentially pertinent, from here: https://www.bbc.co.uk/programmes/articles/1BdGTSWrgGHQ1DMhpVJhDcf/nine-secrets-to-improve-your-relationships
-> This shows that it was natural and normal for her to seek some support elsewhere. It just got complicated; that happens.
So this is how I see it. I'm not a therapist / counsellor / psychologist, I just have some knowledge and some experiences.
What do you think?