Firstly I’d sort my water quality if it was 15ppm.Secondly I’d bin off crap jobs like that. Especially when they start complaining. They’re lucky they’ve even got a window cleaner.
"Told leave the front we’ve re done them ourselves.."Mmmmm, bit odd, what customer would go to the bother of calling back the window cleaner to redo the windows, then do half of them themselves, especially leaded windows... that would fit in with the theory that kids with water pistols had been playing in the back garden.
If its only one job out of the hundreds that you do i would not worry about it. i would clean the next time with the water at zero and if this does not sort the problem blame the old lead oxidation on the windows dump it and move on to the next job on your list