Some good advice already given on here. Seems like there may be a 'pandora's box' here waiting to be opened. Just make sure you really want to open it. I agree with ModelT & Dollie in that the team approach is best. Do you really want her to go solo? Wouldn't you get more out of it if you were involved too? Maybe tell her you'd like her to do it but you really want to be there - if only to watch and give her support. My wife has had other men, but neither of us liked the idea of her going alone with another man. The big turn-on for us was that once we'd both agreed in principle to her infidelity we laid the plan together and enjoyed both playing our roles in achieving its execution. Maybe if you tell your wife you want to be there and watch her suck this guy's cock she'll find that even more stimulating. It might be that the man isn't into it - but given the option of having her suck his cock on your (both of you) terms or not getting it sucked at all I think he'll ovecome any qualms pretty quickly
Then organise a time and venue that suits you all.
It may take longer, but then that's okay, I find that a huge part of the thrill is in the gradual progress of the project, the setting in motion of the scheme and the rising feeling of anticipation as you move it towards the moment. Most important, though, is the trust between you - if it's not something that you both want equally and you both trust each other that there'll be no comebacks on either side then it's a non-starter. Good luck and keep us posted