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.

Deadlines and commitments, what to leave in, what to leave out...