I've been working on my windlass recently. The motor wires directly to the negative battery terminal. The switching happens on the positive side.
If your guy was touching the lifeline and the windlass and got a shock, I'll bet the positive was from the lifeline, and the negative for a circuit was from the windlass.
So the wires in the lifeline is the thing to check first. Check the voltage from the lifeline to the motor casing (I'm assuming there is some connection from the motor negative to the casing, which may be inadvertent).
Also, if one button works and the other doesn't, it could be the button, or it could be the control box. My control box welded the contacts on one side for a runaway condition. After "repairing" the control box, the up button works but not the down button because some of the wires from the solenoids to the control button spade terminals inside the box came free and I had to resolder them. Apparently I was successful on the up button but not the down.