In poly, people are free to negotiate what ever rules and guidelines that they want. It is pretty typical for the person who is least easy with poly to want to make a bunch of restrictions and rules to protect themselves. This is natural and perfectly understandable. Sometimes one or both partners have veto power over potential lovers.
But if your wife loves this other man and the set of rules and restrictions that you place effectively mean that she can't see him (or can almost never see him), then she will resent these rules, as surely as if you simply insisted she becomes mono. If she really loves this guy, and you want to do the poly thing, I think you will have to make time for them to be together.
You want the Other Significant Other (OSO) to be a source of joy in your relationship with your wife, and not a breeding ground for resentment.
It is true that there is a chance that she may decide the OSO is the guy for her and leave you. There is also a chance that if you put too many restrictions on their relationship she will get frustrated and angry and develop a real reason to want to leave you.
One advantage of poly relationships is that they are stronger because they better meet the needs of the parties involved. My wife is LESS likely to want to drop me for greener pastures because she can have me AND the greener pastures.
A lot of threads in the 'new to poly' category discuss these issues. I suggest you read a bunch of them to gain a bit more perspective.
Best of luck to you and your growing poly family.
Warm regards, Rick.