Lately, I've been looking into guns for my arthritic mother-in-law and trying to think this topic through. I've put some thoughts together about guns for arthritics in this article:
A Guide to Handguns for Arthritics, Carpal Tunnel Sufferers, and Weak Wristed Shooters
Right now, I'm thinking a simple full-sized, full-weight, revolver with comfortable grips and a somewhat weaker caliber would probably be the way to go.