I'm sure the ATF is going to drop everything and get right on that! His only chance of getting it back, rests with your willingness to return it. Unless somehow, your wife failed to return it in accordance with their divorce decree, or he previously reported it stolen, he's SOL. It's no different than any other property. No, I'm not a lawyer but I've had to deal with similar issues.
Bingo.
Substitute the word "guitar" for "firearm" in your musings.
Property is property. Let him call the ATF... your wife has committed NO CRIME!
If you are not actively engaging EVERY enemy you encounter... you are allowing another to fight for you... and that is cowardice... plain and simple.
