I would suggest a 4" Colt Python loaded with +P .38s or a 4" Ruger Redhawk .45 colt.
Seems strange on the surface, right? But those two guns have the lightest DA pull I've found, lighter and more manageable than the S&W revolver DA pull. With her arthritis, that might be important.
Among smaller guns I'd go with a Ruger SP101, then spend some range time changing springs and testing for reliability. Find the lightest hammer spring that gives 100% reliability, then match it with the lightest trigger spring that does always every time return the trigger for a second shot. Caliber? Well, I'm partial to the .357, but it might make a lot of sense to go with .327 Federal and if the recoil is too much, drop back to .32 mag ammo for it.
Tom
