Hello!
She can do range of motion exercises involving her legs and hands by sitting and also tell her to do exercises involving movements of legs and hands while sleeping on her bed.
She can do ankle exercises, knee exercises and simple stepping exercises while standing at one position.
She should take a go ahead from her surgeon before she starts some outdoor activity but for sitting and standing and sleeping exercises she can start.
Physiotherapist is an expert to teach these exercises and movements and tell her to increase the frequency and time put on exercising to cut down her weight. Tell her to eat properly and prepare a customized diet chart with the help of a dietician.
Take care!
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Does she have permission from her doctor to exercise? Walking is always a good way to start. She will need to start slowly - walking short distances, maybe 2-3 times/day in order to build her strength; as her strength builds, she should be able to walk further/longer.
She could also try simple "chair exercises" such as leg lifts, some arm exercises, etc. You can google a certain type of exercise and usually come up with quite a few.
But, again -- please make sure she has permission from her doctor as you would not want her to do more damage by doing things not recommended for her medical issues. Some doctors will even give you sheets of paper with exercises that are suitable for certain conditions.