I am a chair user and have been for 14 years now. My rib is set up for use by me. i wouldn't bother strapping a chair down and sitting in it on a boat, salt spray will wreck the chair and they a fffffing expensive for a good one. a bench seat is the best for a chair user, but flat topped jockeys are also useable, grab handles are a big help also.
With regards to slamming the hull down from waves and jarring yourself, lets just say it teaches you to drive it quickly
i've been out pretty rough stuff without any problems.
i've pm'd my number if you want any advice. Do you know what level your friends spinal injury is eg T12 (lower thorasic) or T1 (top thorasic) as the balance of people with different levels of injury varies.