My mother has RA in all her joints and also has Fibromyalgia. Since she has no cartlidge left in the knees is it still good to exercise.

She should see a rheumatologist or anyone who specializes in these diseases. No excercise is not good - she will "freeze up." But with carlidge gone, exercise is no doubt painful (this happened to my mother, and she was too frail for replacement surgery). Please try to get her to see a doctor about how to best use her joints.
Carol