Looks like you've received some pretty good advice so far Waders. A couple of points of clarification:
My 70 year old mother, who has a slight touch of arthritis, but is otherwise in fine health...
....one thing that you've not addressed is your mom's size/stature. It's usually only of peripheral importance except in this case due to the arthritis. Arthritis will only get worse and more debilitating in that the diminishing strength in her hands and flexibility of her fingers will cause functional shrinkage. Therefore a gun that she can comfortably manipulate and that fits well today, may not be suitable in the near future. If she's got pretty good sized hands and well developed forearm muscles, then something like a K-frame will probably be great. If a K-frame is just passable for her now, it probably won't be in the near future and you may want to consider a J-framed sized gun.
...She is not terribly likely to carry concealed (but she already went and got her CPL, just in case), and she will likely practice about once a year or less once the honeymoon with her new gun is over.
...kudos to you on your realism...I think alot of defensive weaponry is purchased with more fervor than rational anticipation. The word "likely" enters the issue of probability. It sounds low, but possible. What features would you/she want for concealability that would be irrelevant for a "night-stand" gun? Probably barrel length---possibly a shrouded hammer---maybe weight. If you genuinely think that she won't practice with the gun after a "honeymoon" with it, then the advantage of the added sight-radius for a long barrel is fairly irrelevant---ie. she won't have had the skill developed to benefit from the added precision. Barrel length then become critical only as it affects the performance of the cartridge and the handling dynamics (pointability) of the handgun and that (pointability) is a very personal thing.
I'm thinking a revolver with an exposed hammer in .38 Special. I don't know how she would deal with the "complexity" of an automatic. But, I think she'd be OK with the recoil of a .38--she's a tough chick.
....just curious as to why you feel an exposed hammer would be beneficial? Granted it allows for single action shooting and a better trigger pull, however that's a skill set that will demand practice---while not truely a "fine-motor-skill" it's adding an element of "complexity" to the equation. A dao weapon will eliminate the complexity and is pretty much good to go with only gross motor skills---takes some practice to be as accurate as with a single action trigger pull, but at bad-breath distances to the target, may not be a critical factor.
....just thoughts to ponder.
Sounds like so far a 3" or 4" K-frame is leading in the suggestions---sounds like a good idea as long as your mom's got the hand size/strength that is more than marginal--again, her hands will get weaker and functionally smaller with arthritis.
Also sounds like there is merit in a shrouded J-frame if you feel that she just might drop the gun in her purse when she goes to play bingo and/or has marginally sized hands/strength.
Of course, I could be wrong, JMO---YMMV.
