Argument: Windscreen broke due to age, not due to buffing.
The windscreen was already more than thrice past its design life. It had to give way at one time or the other.
Buffing is useless anyhow. Glass loses its water-beading property and acquires water-smearing property.
The screen broke at routine points of strain-collection (don't know the technical term for it). Doesn't look like a problem caused by buffing, but one caused by old age and material fatigue. Failure may have been accelerated by one or two weeks due to the stress of vibrating motor.
Why should he have paid for the new windscreen when it didn't break due to his action? It was just the straw that broke the camel's back. The other load wasn't caused by him.