I agree completely with Phy. I don't believe in the spiritual dimension of karma, per se, but it's undeniable that when you invite negative things into your life (dishonesty, participating in the damage/destruction of another's relationship), you invite bad things into your own (further dishonesty, drama).
You say you guys have done lots of flirting over the years. That being so, you and your wife will have plenty of other chances to explore outside relationships if it's not right with this particular guy.
If you know that he's willing to pass you in the office and smile at you, while thinking to himself that he's a partner in deceiving you, how can you trust him to, say, be honest with your wife about his std status? How can you trust him at all?
If I were you, I would ask my wife to come clean to him. And if that means she doesn't get to sleep with him, oh well. There are other fish in the sea. I would also recommend not getting involved with a cheater at all. To put it another way, if his own wife, his life partner, can't trust him why should either of you?
But maybe bringing up poly to him will give him a chance to think about doing things another way. Maybe he hasn't done the internet exploration you have and doesn't know of it -- many don't. Maybe this could change the whole course of his life and his marriage in a positive way. On what do you base the idea that he wouldn't be cool with it? Why not give him the opportunity to know what he's getting into?
Poly is defined by some as ethical nonmonogamy, and by others as multiple loving relationships. This ain't ethical as it stands, and, as Phy pointed put, your wife can hardly have a "relationship"with this guy if he thinks he's a secret and doesn't know what the real deal is in her life.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.