Why is it silly?
What if it rains heavily, thunder, lightening, local traffic etc etc
I doubt if a soft brush against the window with some water is likely to induce a heart attack.....if it does then clearly time is up for that animal
My dog knows when my car pulls up outside the house. He can distinguish the sound my car makes from other cars who may stop outside my house. This is a fact. When I come home, Wor Lass says my dog runs to the door even before I've turned off my car engine.
So if someone is pushing a brush around a window on 'his' house, he's going to know about it; big style. He'll know you're in the garden by the click of the gate.
It would stress him and if he was ill, I'd choose the welfare of my dog over any window cleaner.
I'm a window cleaner, a dog owner; and that's how I'd see it.
However, I must point out that one of my pet hates is being told, 'Not this month, mate!'.
That means 'never again' in my books, but I would agree that a sick pet is a good enough reason to be turned away; yet return the next time they're due.
Aren't we meant to be a nation of animal lovers?