I'd take her to a trusted gun store with a range. At a good store, the staff should go out of their way to help her find a weapon that works for her. I'd buy her a session or two of training with that gun at the same time if you aren't confident/qualified to teach her, or if you think she'd feel more confident with someone else (i.e., a "professional" may not have more experience but she might believe they are, and that might make all the difference).
For my mom? The best fit would probably be a reasonably lightweight 20 ga auto with a correctly fitted, recoil reducing stock, 18" barrel, a slightly extended mag tube (same capacity as a revolver), and a slightly large fibre optic front bead to make it easy to aim accurately. I know some people hate the idea of a shotgun as a home defense weapon, but they are easy to understand and use.