Pcm
The fact that the voltage drops at the same time makes me suspect the PCM (behind the battery in the fender) It reads the crank sensor and then decides whether to charge the battery and put transmission into OD, etc.
Could be Crank sensor, but I would suspect the PCM or connection thereof.
I am assuming that your buddy has late 91 intercooled truck. The earlier ones had discrete voltage regulators and I believe no OD.
Ken