What exercises should I do when my knee is swollen with water?

My mother has been going to the hospital continuously because she has knee pain.

The doctor told me to regularly receive injections to drain the fluid from my knee and advised me not to walk if possible, except for cycling. So, I only sit and do nothing other than cycling once a day.

My mother is in her 80s, and I'm worried that if she doesn't move like this, her muscle strength will weaken and it will become even more difficult for her to be active.

Is it really better not to move when your knee is swollen with fluid?