Generally speaking ,No harmful effects for using hot water bag . Can you please elaborate on the condition of your mother to exactly tell you what could be done . You can consult with full history to discuss this if you wish . Please do mind that , is she is not having any sensation in her lower limbs seen in some type advanced type of cancer patients or a paralysis , then hot water bag sensation may not be felt and chance of burn injuries will be high so please me careful regarding that .

Yes, you can use hot water bag for leg pain and weakness as it might give some temporary relief but if it is weakness alone it is better to discuss and get advice from treating doctor as different chemotherapy agents have different effects and always hot bag might not be helpful uniformly for all chemotherapy agents.