I think once he has cleaned them himself, he'll realise what a complete pain they are to do; especially working off a ladder, at height, for an hour or two.
A while back I quoted for a fsg job (the back had a large conservatory) but the customer wasn't happy with the price and said her husband would do them, and he had been looking online for a pole with a brush, that plugs into their outside tap.
8 weeks later I returned to do the windows and he had indeed cleaned them, but the wife said he would never clean them again as it was such hard work!
That's why the price is the price.
I only clean them on bungalows now...(fsg and sp) and I have to be in the mood!