Spring loaded samurai swords that pop out forcefully at various angles in her vulnerable blind spots as well as forward, sideways, and backwards.
Trying to be practical, I'd say a S&W L-Frame .357 loaded with .38 special +P Hydra-Shocks. The L-Frame is a 7-shot revolver. With a 4" barrel she could keep it stowed in the wheelchair without being in the way. .38+Ps won't kick as much as .357s for her fragile old wrist bones, especially in the L-frame, which is heavier than a smaller snub-nosed revolver. I suggest a revolver over a semi-auto because it's simpler to work and maintain, and she can keep it fully loaded without having to worry about working the slide or keeping a safety on.
Assuming she has enough strength to pull that heavy double action trigger. My mom isn't anywhere near elderly yet, but she's gotten so that she doesn't have the strength to pull the trigger on a revolver, or rack the slide on an auto loader. When I was over there for Christmas, I took her to a gun shop and let her fiddlearound with a few pistol caliber carbines. She liked the Beretta Storm.
That's a good point about the trigger pull. It had crossed my mind, but I didn't have a good answer. She should be able to thumb-cock it, though, right? My mom has a .22 lever-action carbine by the door. But I don't know what she's gonna do about it while her broken arm is in a sling. btw, here she is chasing the squirrels out of her bird feeder with her BB gun:
