I don't know what the "new" 172s have, but the old 172s have a interior-light rheostat that's on the left side of the left hand yoke, just left of the ignition/mag switch. The switches in the middle between the yokes only control external lights and pitot heat. (It's actually 2 rheostats co-axial. one for interior light, and one for instrument lights)
A right hand passenger can reach it, but I'd totally understand him not wanting to mess it up either.