Hey there and welcome.
I think that it is great that you yourself feel that you could be happy with the joy your wife has found in this encounter. As you may already have read, if you really want to do poly, the handling of multiple loving relationships, dealing with unpleasant emotions that may cause jealousy is a big topic among the beginners and poly-experienced people. So congrats on that.
But, (and this but is serious), from what you told here, you aren't doing poly at the moment. You are on the best way to participate in cheating. And that's a severe fault in this process. If your wife engages herself with this guy, she will NOT be in another relationship with him, that could be seen as poly, because the honesty on all fronts is missing here. You could be about to invite severe drama into your marriage and the relationship of this other couple. You can't write their relationship off as something that doesn't concern you. Because if you know that he is cheating on his spouse with your wife, you acknowledge his behavior and take an active part in it. (If we want to stick to the facts: Your wife in an active and you in a passive way.)
Think about the consequences and ask yourself if you really want to hurt another person this much. Ask your wife if she wants to play a major role in something like that. Have a look around the forum, there are numerous threads about cheating and the harm it does to all involved. Broken trust is one of the most devastating things a relationship could face. And this guy is about to invite this threat right into his life. Maybe you could ask what this says about his character/prudence and how favorable it is that your wife involves herself with such a person. (I don't know him but he seems to be all in for this step.)
It's great that you and your wife feel ready to explore new relationship dynamics and deepen your bond along the way and share new and important parts of your life or whatever fascinates you most about the idea. But doing poly means you are connected to those people you invite into your life in an emotional way. It goes beyond physical contact, you want to form deeper relationships with those people and you will participate in their life as they do in yours. And you should want to be as responsible as you are in regard to your spouse. That's why cheating shouldn't be in your book when you think about being/doing poly.
So maybe, if you want this experience to be a lasting and good one, you should sit down with your wife, the man she is involved with (as the persons who are already in the picture) and later on the way his spouse to see where everyone stands in this process of opening your marriage or even the relationship of the other couple. It could evolve to something great and mind-blowing (speaking from own experience here
) but there are some traps you should look out for.
These are just my thoughts on your situation, but I hope you will find a way to handle this new experience and reach a successful solution in the end. Good luck.
Facts: 30, female, bi, v-type relationship with Sward (husband, straight, mono) and Lin (boyfriend, straight, mono), poly-fi and co-primary.