Where I live it would not be possible to clean windows with a jet wash and tap water, the tds is around 400. But whats the tds where you live, also if the tds is fairly low and if the impurities are made up by dirt that won't stick to the windows then it might just be possible that she has cleaned the windows with a jet wash, very unlikely, but not impossible.
Did she say she cleaned them and then not pay you the money, obviously if she is telling the truth then you could understand she might be slightly cheesed off, however most of my experiences of people doing this kind of thing are that they're trying to pull a fast one.
Seeing as it looks like a reasonable earner, it might be worth giving her another chance and then you might be able to quote the famous words of president Bush, "There's an old saying in Tennessee — I know it's in Texas, probably in Tennessee — that says, fool me once, shame on — shame on you. Fool me — you can't get fooled again."
www.youtube.com/watch?v=eKgPY1adc0ASimon.