If the ball count stays at 1 and won't advance, I would check that unit for free movement and operation. They do get mighty sticky over the years with the old grease that was used.

Sounds like he doesn't. First things first. Order a schematic and manual from Pinball Resource. Steve Young can help you identify the parts and will happily throw suggestions your way.

There are typically two latches that make the unit increment, or step, properly. Sometimes they can get sticky and cause the problem you are experiencing.