Don -- my heart is out to you. You wrote that you were informed re his wife's demand. None of my business, but (I assume you reside in driving distance?) I'd suggest an in-person social gather -- e.g., take all family our for a medium-priced restaurant meal. (Important that this is in "neutral territory"!) Then, as part of dinner-time conversation, simply state you're getting on in years and for so many had the firearms as YOUR recreation. They've accrued in value $$$-wise, and it would really give immense pleasure for you to -- eventually -- hand them down to your son.
In no way be confrontational -- even to a flat "NO WAY!!!" -- at least pretend it's not the end of the world to you if she enforces a ban...
BUT, I do believe you are due -- from HER mouth -- her feelings. In can be added the "R"-word: respect. If she has credible reasons for her feelings, you can even voice. "that really sucks, but I respect you and your reasons." Life -- including hopefully a happy marriage betwixt your son and her -- goes on...
On the other hand, years back a buddy named Charles married a gal who also had a zero tolerance for guns, and he rented a table at an upcoming show and sold most all. (I even bought two
). Anyhoos -- 27 months later he was again single -- and, without his firearms. (I gave back the lever action (a Model 36 Marlin) and handgun...)
BEST!
geo