This can be cleaned without power washing. Like Mike said, 3-4% solution, let it dwell then agitate with a soft bristle brush.

A little more work, but if your friend is anti power wash, this is your way around it.

I would use the pressure washer first, but only if you have a surface cleaner, or maybe a turbo or "spinner" tip. The turbo is good for exposed aggregate, as it gets in all the nooks and crannies, but be careful if it starts to eat up the surface, and it will send debris flying everywhere. I would use the surface cleaner first, and resort to a turbo only if needed. Also, if you have a hot water machine, I think hot water always helps.

Then apply the SW chems. This will give the instant results, and no stripes. Also, the surface cleaner divides your pressure in half, and it is never concentrated in one spot for more than a fraction of a second. The reason I SW after the surface cleaning is that it does not spray the chems all over the place. A good medium stiff broom would help, too.

On the other hand, if you are pushing the green, low emissions, SW only idea, just ask your customer to be patient. Just let them know the results may not be immediate or perfect.