I think the behavior that your Mother is exhibiting is unfortunately not that unusual for many seniors. Although I don't think there really is a canned "solution", I do think there are things that would greatly improve your Mother's situaton. I would suggest trying to encourage as much activity as possible for your Mother. She may not be very receptive to the idea, so I know it won't be easy but perhaps if she is made to feel like her involvement would really benefit another, less fortunate, she may be more willing to get outside herself for awhile. There are usually senior centers that have programs that handle just this kind of problem. Most of the time, there are jobs and projects that seniors can sign up for that not only provide income but make them feel productive and useful as they once did.
If you do feel that her condition is more severe, she may need a mild antidepressant like zoloft or something but this would mean consulting her physician which may be helpful at this juncture anyway. I wish you luck with your Mother, do try and utilize as many outside resources as you can in your community as they do exist and they are very familiar with this kind of problem. She is lucky to have someone that cares!