You need to decide how you really feel about this. Your wife may be legally entitled to the rifle, but it sounds like you may have to get involved in a legal hassle to keep it.
I don't care about the AR-15, but I don't want this to escalate into a problem for me and my wife owning our current firearms and any future purchases.
If you
truly don't care about the rifle and you
truly don't want a hassle, give the man his rifle back. (yes, I know it might
legally be your wife's rifle now, but I'm sure you can understand why he feels it's still his.) Don't send it directly, that would be illegal, but via an FFL in NY. If he won't agree to using an FFL, tell him to pound sand.
Plus there is the principal of not giving him any satisfaction because he suddenly wants to make something an issue.
This is completely different from your previous paragraph. I can understand the principle of not being directed by jerks. How much hassle are you willing to go through to keep a rifle you say you don't care about just because the guy is a jerk?
If you decide to keep it, look at your wife's divorce papers and see if there is a clause about left property. If there is, tell him you have the rifle, it's securely stored and it's your wife's property now.