I also agree with Harris, I dont think lack of protein in the issue. As always the standard book answer is about 1.2- 1.7 grams of protein per kg of body weight. This of course will depend on the person and training goal. Taking in to much protein of course will dehydrate someone and is hard on the kidneys if you consume to much at anyone time. Dehydration is already an issue for older population. So I agree get a Doctors opinion and continue to work legs. If strength is what you are looking for try increasing weight and lowering reps as tolerable to client.