I've got some mild hand arthritis I'm trying to keep from getting worse. My 638 is strictly a wadcutter-only gun nowadays.
My 640 Pro (all steel) with +P .38 Special loads is just fine and is incredibly mild with wadcutters. I wonder if the .357 frame LCR, an all-steel J-frame, or an SP101 is the ticket for her. Start with wadcutters and work your way up the power scale until you find the edge of her comfort zone.
Or just stop at wadcutters. To get a .38 Special JHP to expand, you usually have to bump the velocity up quite a bit.
The 640 Pro's sights make it shoot like a much bigger gun. The Wiley Clapp SP101 is also a very nice gun.