Starting with the simple stuff, even though batteries are newish, I'd get him to put new ones in first,
could be as simple as a bad battery or bad connection.
Cheers,
Lindsay

+1 on this
does he have a remote battery holder or is it the original one on the board?
I would get him to check whether he is getting the 4.5v at the end contacts.

you mentioned that the flick of the switch trick does not work, get him to unplug the plugs for the switches on the mpu board in case there is a short or it has a faulty diagnostic button.

they are the basics i would check for.