The energy (heat) of reiki will certainly help with rebuilding muscle tone. I've noticed it in back muscle where the back had problems (spine deviation to the right) with muscle loss on the left side. Just a few sessions (4-5) have helped to stimulate balance and lead to some improvement on the weak side.
You can do some gentle massage on her ankles to help her wrists if she finds massage to the wrists too strong.
Body parts correspond:
Top of the feet = back of the hand
Soles of the feet = palms of the hands
Ankles = wrists
Legs = forearms
Knees = Elbows
Thighs = upper arms
Hips = shoulders
This helps if for any reason you cannot do a massage of the affected part due to wounds ore intense pain
You can do reiki on any part of the body, even if not directly affected, the energy goes on its own to ehere it's most needed.
Best entrance points are: feet, knees, shoulders and sometimes elbows as well. This really depends a lot on the person. For men is better the knees and for women often the feet is the best entrance point. The most complex entrance pont is the head. Better not use it unless that person has a strong crown chakra, or it will take too long to get the energy in, due to stress and worries blockages.