If a band broke, then that isn't pressure related. Reason being is the piston that actuates it will only move as far as the bore allows it. Therefore it moves forward and stops, in the process it pushes on the band. It can't surpass the bore, it shouldn't do much by applying faster based on higher pressure.
Was this the forward apply band? If so, likely he inspected it and reused it. Which..in a trans known for snapping the stock one over time.. let's say.. it shouldn't be reused for the cost of a new one.
