No, never proceed with an affair. To make any of these fetish lifestyles work, it takes open communication and blunt honesty and trust. None of which you are proposing.
Check out /r/hotwife and ask this question in there. Hotwife is about women who fuck other men while their husband either watches, records it, or sits at home and waits for updates via text or when the wife gets home.
What I will caution you about however is the Dom/Sub lifestyle with another person. See, the whole Dom/Sub relationship is that the guy is her "master" and she is his "slave"..... If you can't see the future pitfalls of you, her husband, not being her master, then you need to take a gigantic step back and re-think things.
If this other guy is her master, then she will have to do whatever he wants. Yes, it will start out sexual, but it won't end there. It will then morph into more and more devious things which can include straight leaving you to please him. Remember, you won't be her master, he will. She will be his property, not your wife anymore. Again, if you can't see the pitfalls, then you need to take a time out and re-think things.
You might have plans for a lovely weekend together. But then she gets the text from her master telling her to stop whatever she's doing and leave to come to his place (or wherever).... the nature of their dom/sub relationship will actually have her leaving you (her husband) to please him (her master). Think of it as a dog with its master. The master is #1, everyone else is secondary. You, being her husband, will be secondary. Sure, it won't start out that way, and they'll both claim it'll never get that way, but that's because you're new to this all. It ALWAYS turns into that. It just does. It's like counting. 1, 2, 3... when you're young (or naive in your case), you don't notice that numbers go on to infinity. You may think numbers stop at 10 because that's all you can count to. But soon, you'll find out that the numbers keep going. They never stop. The same is with Dom/Sub relationships. It starts off as just harmless sex, but then it goes farther, then farther, then farther. Next thing you know, it's well beyond what you ever discussed or thought possible and you are starting at a woman who no longer respects you and who is someone else's property to do with as he pleases. Again, this might be find if you are her master, but you won't be. Someone else will be. That's why this is a HORRIBLE idea for you. 10 out of 10 it ends in bitter divorce or a relationship that is so damaged that it'll never be the same.
Now, again, don't have this guy have an affair. Just post on a swingers website or fetish website or hell, even craigslist and find yourself a "bull" to fuck your wife. They are everywhere, and best of all, you won't be destroying a relationship to please your fantasy (which again, has a lot of pit falls seeing you won't be her master, another man will).