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.