The heading sensor has nothing to do with the spot lock function. The Ultrex will spot lock just fine even if it's not installed. The jog feature however, will not work without the heading sensor, but all the GPS electronics for spot lock are contained in the head of the Ultrex.

I would bet on his "boat scale" (as mentioned above) still being set to the default of "0". Tell him to turn it down to -2 and see how that does.