Unfortunately he probably has the original 35 ampere which barely keeps up with a completely stock system.

Grab an alternator from a '80's W123 or W126. You might need to upgrade the main cable to a larger gauge but it's pretty much a "plug and play".

Anything newer and you run into pulley problems.

__________________
“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now