If anything close to poly is to come out of this affair, then it would start with the three of you sitting down together in a private place away from anyone's home, a neutral place. Boundaries should be explored, feelings expressed, etc.
From what it sounds like, he may not agree to this, and she may not either, but that could be your ultimatum. I would think there would be big changes for all of you if you all met. I try to get that part done as soon as possible to make sure everyone is comfortable and feels loved and respected.
I am bumping this as an example of the kind of advice newbies on this forum used to get a decade or more ago:
"If your committed partner is starting a relationship with someone else, you must sit down with your metamour and talk over the expectations for the new relationship, going forward, and as soon as possible."
In this case, Redpepper is strongly suggesting that RaleighGuy must "sit down" with a guy, an an old crush of his wife's, who she herself doesn't want to meet in person, a guy who lives 2000 miles away. This not only indicates this old attitude of "you must meet any potential metamour asap, to vet them and approve of their intentions" (as if you're an old-fashioned father), it also shows a lack of comprehension of the fact that this new guy lives 2000 miles away, and the wife doesn't want to meet him, he doesn't want to meet her, and no one can really afford a plane ticket anyway.
Bleh. Bad and impractical advice.
No one ever "needs" to meet a potential metamour. Perfectly good poly relationships can exist in parallel. A partner doesn't need to trust their metamour. They need to trust their own partner and respect that partner's choices.
Approaches to polyamory have definitely evolved in the past decade or two.
In this case, yes, the wife is having an affair, because her old flame's wife doesn't know he is falling back in love with an old gf. That's the only problem. It isn't polyamory, it is a cheating affair, and the guy is deceiving his wife, which could lead to problems, should she find out.