You can't makeyour mother exercise and shouldn't feel responsible if she doesn't listen to you -- or her doctor. But you can refuse to let the subject drop, even if it makes her uncomfortable, and that's the easiest path to take.
Remind her frequently that her medical professionals are recommending exercise, and she needs to try to follow their advice, no matter how hard it is. If possible, ask the doctor to talk to her about it again. The more she hears the message -- and the more people she hears it from -- the more it will get through.
One strategy is to have her doctor prescribe physical therapy. Then the physical therapist can get involved and recommend some gentle exercises to try at home.
If your doctor's staff can recommend exercise classes at a nearby wellness center or health club (many of which are offered free of charge for cancer patients), the buddy system works really well. Pair your mother up with another group member, and they can encourage each other and gauge each other's progress.
Be as encouraging as possible, and offer lots of choices. Your mother might like a beginning yoga or Tai Chi class. Acknowledge that taking a class might be scary for her the first time. If your schedule permits, offer to go with her to the class and participate yourself. (There are also some great exercise videos to try at home.)
The more you can make it seem like it's your mother's choice to get moving, the better. Cancer patients often feel powerless and out of control, but making choices -- even small ones -- can help your mother feel more in control.
If your mother is truly becoming inactive, you may have to stand firm and use tough love. There was one point when I was taking care of my father when he said he couldn't stand up to use the restroom and wanted me to bring him the urinal in bed. I said no, the doctor says you have to stand up and use your legs, or you'll lose the use of them altogether.
Bit by bit, we worked on sitting up, moving his legs, and taking a few steps. Taking three steps was a really big deal, and we celebrated with champagne when he got to 13 steps. Work on getting your mother moving one bit at a time, and acknowledge any progress she makes as a big accomplishment.
You can also use little rewards and motivators. With my dad, I kept some Sea Breeze astringent close by, and when he got sweaty from walking, I'd put some on the insides of his elbows and the backs of his knees to help him cool down. Sometimes I'd also give him a little leg and foot massage afterward. He really looked forward to these "mini spas," and it gave him something to look forward to after his hard work.