Since it has a gas motor; it would have had a 6 volt generator and battery and wired + ground from the factory.
So help us out........
Is he using a 6 or 12 volt battery?
Does it still have the generator or has it been converted to a 12v alternator? How does he have it wired + or - ground.
If you really think something is draining the battery over night you do not have to wait for dark and start looking for hard to see sparks while taking a wire loose.
Take one of the battery cables loose. Install a test light between the battery post and the battery wire. If the light comes on you have a drain somewhere. If you remove the drain the light will go off.