The problem however is that the indicator is not pointing anywhere on the left side of the meter. I know from experience that calibrating and turning in circles should cause the meter pointer to swing 360 degrees. Now it will swing less than 180 degrees and back up.
Most masthead wind direction sensors have two moving coils spaced 90 degrees apart; one gives a value representing the north-south axis, the other gives a value corresponding to the west-east axis. Your problem sounds like the west-east axis output isn't there, which first of all suggests connections or cabling.
I assume you've already checked and cleaned the connections on the back of the display unit.
When you send your friend up the mast, first have them open and inspect the connection to the MHU, clean any dirt or corrosion, add a little contact enhancer, and put back together. Immediately try the unit to see if that helped. If it did help, you can do a quickie calibration by having your friend manually turn the wind direction sensor through 360 or 720 degrees while they're still aloft.
If all this hasn't fixed the problem, the fault is most likely in the MHU and your friend should bring it down.
{edit - ah, I see you've taken the mast down. ]