Thanks for sharing the solution. Nice to hear about kids getting interested in troubleshooting. It's fun work. I like working on the games just as much as playing them.
In addition to ringing cables from the MPU to the playfield, you could also give him some background on the switch matrix and then show him how you can connect a diode from the column connector to the row connector on the MPU and see that particular switch register. If the diagnostics show the switch closed when you do that, then you know the problem is in the harness or on the playfield. The switch and lamp matrices are really interesting and it's a great practical application of diodes now that he understands what they do.