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.
Excellent suggestion. My S&W Model 60-10 has Crimson Trace *and* a high-tech fiber optic sight on it. In low-light conditions, the CT makes aiming a snap. Unless her vision is really worse than mine (not likely since she can read), the same is likely to be true for her. In daylight conditions, the CT is useless but *if* my target is far enough away to require actual aiming, the fiber optic sight works very well. Just be sure to get it adjusted for the distances that you expect to shoot.
As before, though, don't just take my word for it. *Try things out* and let her make the decision. (BTW, PM me if you live anywhere near northern Nevada. I'd be delighted to talk to your wife about this issue.)