I suppose the customer could be gently and politely be reminded that outside walls of houses are usually designed to keep the water out with persuasive words like 'your a thick moronic idiot'.
I just dump customers like that without an explanation as I've found that they often let these weird ideas fester in their tiny brains before allowing it to resurface at the next clean. Then after a while you find yourself not wanting to do their windows anyway because of the repeated original complaint.
I agree with Cozy, it's time to trim the herd.