to me her vision problems are a major consideration. Can she see a laser dot at the defensive distance she would be firing. If she can then I would recommend a laser or a flashlight which is centered at that distance.
As far as the weapon is concerned 20 gauge youth shotgun might be a good choice. If the 20 is to heavy then you might consider a 410. At close range it performs very well with buck or slug.
pistol wise a quality 32 or 380 with the suggested sitting systems.
If that fails, then a ruger 22 auto with the short barrel and fixed sites would be my last choice. A friend of mine who was undercover with the Chicag PD carried and employed a 22 to good effect in the early 70s.