Gas pressure factor wrong on a 2 psi service. Therms overstated for years.

This one took three passes to find. Laundromat with elevated pressure gas service for the dryers and
the water heater. Meter is a diaphragm meter measuring at delivery pressure, and the utility applies
a pressure factor to convert measured cubic feet to standard cubic feet before billing therms.

The factor on the bill corresponds to a service pressure noticeably below what the regulator is
actually set at. I got the regulator setting from the client's mechanical contractor and it does not
match what the utility has on file.

Direction of the error is in the client's favor on its face, meaning they have been billed for fewer
therms than they used, which is the opposite of what I want to find. But it also means there is an
underbilling exposure if the utility ever catches it, and backbilling rules in this state let them go
back 24 months.

So what is the professional obligation here? I found an error that hurts my client to disclose and
hurts them worse if discovered later. I have my own view but I want to hear how others have handled a
finding that runs against the client.