If mildew and similar are the problem, instead of washing it every year,
how about if he sprays it with bleach couple times a year? You could do
that with a garden sprayer in a few minutes. I'd do it just before rain
is forecast, so it later gets washed off.

There is a product that
Kills algae and mold, then you let sun bleach it. Works all summer long.
Mostly for roof but other things too.

Bleach only provides very short-term mildew relief anyway.