How can I help my mother become strong enough to go on walks with me?

My mother is in a chair full time except to go to doctor appointments twice a week. When she moves she has a walker but I can tell she has muscle wasting going on. I am her live in daughter and also a nurse. She has had PT out but noone has shown me exercising I can help her with in her chair--can you please help me on this?? I want to try to get her stronger by spring so she is able to go for walks with me-- she is 54 years old and has stage 4 lung cancer.