Hi and welcome.
Conductors, especially those that are used to actually carry current, have very low resistivity so no two parts of this cavity will have more than a very few mV across them. So (assuming he can hold his breath or that there are some holes) the person would be fine because no appreciable current could be flowing through him.
If you find yourself in a car on which an electricity cable has fallen, you would be safe as long as the sparks don't cause a fire. Best advice would probably be to stay there until someone rings the electricity company and the power is turned off. Trying to step down to the ground could be risky, though.