I have seen the rear window of an F150 get blown out in an automatic. The truck had been sitting outside all morning in -28 Celsius and then we washed it. It blew out right after the second pre-soak pass.
You might want to explain to the customer that the only way this could happen is because there had to be a chip or small crack in the windshield before they came.
Explain to them that in no way could your car wash crack a windshield unless there was some sort of previous damage that they might not have noticed.
That they can check out other cars that go through the wash and they do not have a crack windshield.
That if they would would report it through their insurance company you would pay them their deductible ($50-10) after the windshield has been fixed but not before.
I doubt your insurance would pay for a new windshield so you would be out a lot more if you had to pay for a new one.
Usualy it is a stress crack that has grown. The tell tale sign of the stress crack is that it typicaly starts at the edge. There is no impact mark. Since all or most is hidden under the moulding it is not noticed until after the wash.
Agreed - need a weak point in the glass first. Also, how hot would the water be when it hits the car after travelling from the tip to car surface? I would imagine it drops quite a bit?