My friend just blew out his knee, and he can't bend it, walk, run, or just about anything. BUT, he is extremely conscious of his weight and staying fit. What is a good cardio exercise that he can do without his leg?
Swimming - Just Don't Kick. Water exercises are great for a few reasons. I like the water because you don't sweat - or rather, you don't feel it if you do. :) But if your friend has blown his knee out, the water may be a great way for him to get some exercise. Have him get some sort of a flotation device to put under his legs and have him use his upper body to do the exercises. Be certain he does all this in a pool where there's an available life guard - we don't want him to accidentally use that leg cause all of a sudden he has to!