Rinse rinse & more rinse.
Yep that's what he has to do.
If it's an add-on service then every job is going to be like a 1st clean. And they take extra time and care to come up good.
Avoid vents at the top of windows after cleaning the window rinse and then rinse some more.
If he's got some cleaning work to do inside then clean the windows 1st so you can get a look afterwards. He may notice a pattern in the types of window that spot.
Wooden stained frames or a bad paint job maybe?
Advise him to take twice as long going around as he does. It will come down to rinsing the glass in the end though.