Dear Sir, the condition of the polio limb will only tell why she has started falling after gaining weight, normally the weight increase is a slow phenomenon and it gives sufficient time for the limb to increase muscle strength, the phenomenon of fall could be due to many other reasons, and that needs evaluation, blindly going for physio, before knowing the cause can even be more harmfull.

She needs to activate and tone up her available thigh and hip muscle. Consult a physio, do TENS for thigh,hip muscles and ask them to teach exercises for the same

Reduce weight Use walking stick Strengthen the muscle by hip and knee exercises

Reduce weight if you can and strengthen the muscles in the legs. Consult physio for exercises