Introducing a disabled friend to photography

I came here to suggest the Manfrotto Clamp also - you can mount a tripod head of your choice to the clamp, which means he'd have control over pointing-angle, within the constraints of being able to see the viewfinder (or not).

If they want something more permanent, you just need some way of attaching a 3/8" male thread to the chair in a choice location and a tripod head will mount to that.

And while movement might be limited, they might come up with more-creative views, e.g. by mounting the camera very low on the chair, shooting while moving, etc.

Waist level is how some medium format cameras were meant to be used. It'll be fine. Not being terribly mobile makes things more like the LF/ULF crowd where it either isn't practical to take the camera far, or the people can't lug a camera far from the car like they used to. I think your friend will be in common company.