You can try an angular and geometric plastic frame with photochromic lenses. This shape of glasses can balance your grandma's round face and make the face look longer and thinner. The photochromic lenses will change to a darker tint in sunlight for protecting your grandma's eyes from ultraviolet rays. We all know too many ultraviolet into eyes will cause cataract. This kind of lenses will reduce the risk of suffering cataract.
For people who have round faces, there are several things you need to pay attention to when you choose the prescription reading glasses for your grandma. In the first place, you should avoid picking lenses that are shaped round, for that will make her face look rounder. Secondly, you can choose some larger glasses for her and black color will be better. It would make her face smaller and thinner. At last, as for the glasses materials, I would suggest titanium eyeglasses frames because it is lighter and more comfortable to wear. Good luck and hope she will like your choice.