I work at a retirement community. People with osteoarthritis usually fall into a downward spiral because it hurts to to move so they stop moving and lose bone density because they stop moving even more.

It is so important to do range of motion exercises because if you don't use it you lose it. You must fight through the pain so that you don't lose mobility. It is also important to do some weight bearing exercise because as you lift weights you improve bone density. A little bit of lifting goes a long way. I have had several people in my classes who were previously classified with osteoperosis, improve from this condition just from a simple exercise program.

Tell your mum to keep it up!