He probably changed the AMP lamp on the dash to a prettier blue one,
causing an idle current increase capability in the rotor.

To be fair, I should say "Might have."

This could energize a hopped up output in the alt, working it toward
death. If it gets hot, it might cool down and stretch the belt less,
if a proper, dimmer alt lamp was reinstalled.

Theraputically, if a resistor was put in series with a too heavy lamp,
it's brightness or warmness (not a dashboard bus dimmer control) and
the alternator excitation and workload might diminish.

Removing the lamp might deactivate the alternator altogether.

It would depend on who jerry wired it in the beginning.

Charge voltage versus specific gravity measurements would determine
this parameter.

Pulley belts need to be in line and adequately wrapped and tensioned.

Removing a bank containing a short battery might cure all of this.
Battery terminal voltage out of circuit lies. A load cell is a good
way to test a battery.

Terry K