No, the starter solenoid grounds through the body of the starter through the engine block and back to the batteries. So it sounds like all of the relays and solenoids are getting power like they should be, so I've got another question for you; did you hear any clicking at the starter solenoid when your buddy was turning the key back and forth between start and on?
If not, then I would rip the starter off the truck and start poking into the solenoid on the starter to see if you've got a partially fused passthrough that might be able to be fixed quickly with some grinding/coarse sanding. After each time you scrub on it, check it with 24v power from your battery setup on the truck with the positive wire on the small lug on the solenoid and the negative on the case to complete the circuit for testing. If you don't hear clicking when you connect and disconnect the wire from the starter body, you'll know your link inside the solenoid is sticking or not moving and more fixing is needed inside the solenoid
If that is not the case and you do hear clicking when you test with 24v power, then the problem lies inside the starter body, then you will probably want to look into starter rebuild links here on the site under the CUCV Helpful Threads sticky IIRC that can reman your starter for you for a decent price, since sourcing a new 24v starter might be quite the pain in the rear end and very expensive if you can't get it through local auto parts stores.