I also have a friend that has no use of his legs, and he rides my quads without any issues. He has fallen off a few times, but he isa really good sport about it.
It makes me happy to hear about people being able to ride without all their parts, now i'm not so worried about having my arms and legs blown off at work LOL
One of the guys I play ballhockey with he is a goalie, he was in an electrical accident years ago lost one foot below his ankle and lost from knee down on his other leg he is an avid quadder and sledder, he has a renegade 800 and a 07 summit 159. I've known him for 10 years now, one of the most active guys I know and one determined fellow when people say you can't do that.