I'd say it's to a non-factory switch to light the machine up, or a fix. Solder it back to where it was taped unless you mean it was literally not connected, then just don't connect it and see how it does. You could do that regardless. Would be very very very easy to spot if something was wrong, that's a very important relay, and the machine simply wouldn't do ANYTHING or light up correctly like it should if that wire was needed.

Since this is his first game AND he's trying to learn, it might be worth the investment to buy a schematic.

That sounds like a fun project of comparing the wiring to the ink.

Just sayin....

Good luck!
faz

On 70's Gottlieb games, any plastic insulated wires are full line current.

I do have this title in my collection. You will find that the 'Blue' wire connects directly to the 'Hold' Coil. This will energise the coil which is then latched through the leaf contacts. In your bottom photo you can see where the blue wire has been cut away from the coil lug.

IMG_4559_(resized).JPG

Yep, without having the game in front of me, that would have been my guess. Right to the coil itself.