In addition to your health, it isn't good for THEIRS to be taken care of by someone who is increasingly unable to do so. She could hire a pet sitter, or look for a good kennel. They vary in both quality and price, and there are some that have people assigned to do things like spend time with the animals that are boarded there.

If she decides on a pet sitter, so the cats can be in their own home, there are some very good, professional pet sitters. Another option, if there's a college nearby, is to hire a pre-vet student. To get into vet school, the students have to have a certain amount of experience working wtih animals in some capacity, so many students who plan to apply to vet school work as pet sitters. Obviously they charge the going rate, but since they're really looking for experience more than money, you get good service at a very reasonable price.