I have several accessories grounded to the metal frame behind the dash, and no problems there--might want to check his wiring on the other end, though.
Also, if he disconnects the switch/relay/whatever he added (all of it) does the light still stay on? If so, make sure the dome light isn't turned on at the switch (yeah, it's stupid, but I've done stuff like that before more times than I can count). Also, might want to check at the door pin switches. My dome light started staying on, I found that the plastic on the switch had worn enough that the switch didn't close all the way sometimes and the light would stay on. Put a little shim behind it, and it works fine again.
--Ty