I think we can say a couple of things; that both motors are turning under power in some times, which indicate the motors and brushes and SCR chopper panel are all working.
I agree that he may have to adjust the steering position potentiometer, but it does seem to be cutting out some of the contactor coils power at roughly the right spots. and therefore, it should run in straight ahead position if the contacts are all making good contact,
I am actually pretty sure he needs to get the manual (or compare to a 'sister' truck) to be able to confirm the location of the power cables for both motors and bus bars to contactors, his symptoms seem similar to those you may get if 2 of the connections are wrong and he is only getting power through the armature on one motor, but it is finding a return to the chopper panel (B-) through the other motors at rest contactor connection.
I would also strongly recommend to do the first step in troubleshooting as per GE, which is to confirm the "shorts to frame" are at least 100K ohms on any of the power cables and chopper panel heat sinks, as he may be getting power through the frame from damaged power cables.