There are a lot of small dogs in shelters that would fit your mother's needs. I wouldn't recommend a Shepherd/Lab/Collie. A poodle is good if you don't mind the grooming requirements. My choice would be an Australian Terrier, Norwich Terrier or similar mix. They are small but sturdy, don't require a lot of grooming and can be good with cats. Cats more readily accept a smaller dog, providing the dog isn't aggressive. I don't think I'd want to get a puppy.
I don't know how old your mother is, or if she has any health concerns. A puppy is a real handful for the first year and she may not want to put up with all the hassle.

Just my 2 cents.