Being 3 legged certainly doesn't stop them exercising, Pebbles could still run as fast on 3 legs as 4, and I regularly gave up catching her, she outran me!! Your friend could feed them on senior food as they are according to pet food makers, and it is important to keep 3 legged cats weight normal, or it can cause issues for them. I have had overweight, underweight and normal cats all at the same time, and they all got their normal food (overweight getting senior food), and when the overweight cats were out of the way, the underweight ones got more food, it can be tricky but it is possible. She could of course reduce Turbo's food intake and/or played more with her, and keep them on the same food, as there are no guarantees they will like light or senior food.