just an FYI... not being sure how far your friends limited ability extends... but I would think twice about "strapping" anyone to a sled, if they're going for any serious mountain fun.
I'm a left, below knee amputee. When I started sledding again (did much as a child) one of the guys I rode with is paralyzed. He's a pretty hard-core motor-head... but he does just fine out there with no additional straps or security.
His legs are paralyzed, but his waist is still somewhat able to help him pivot. If your friend has no waist control... he may not be able to go as hard as my buddy does, but if he's still got some, limited mobility... enough to keep him straight on the seat, with arms helping out, he may be fine without the strapping.
I only bring this up because we've had a tumble or two over time and I can tell you... if he'd have been strapped to his seat, it would have been a much bigger deal and could have resulted in a larger injury. As it is... he slides off, the sled rolls a few feet away and all we need to do is help him get the sled upright and slide on down to it.
If he were strapped under an upside down sled, or forced to remain connected during a tumble... too spooky for my mind.
just another .02 for your consideration from someone who's disabled himself and has ridden with another paralyzed sledder.
