They can't ethically charge him for power they can't show he actually used. At best they're guessing based on current data which may not reflect use during the period in question.
A bill like this could put him out of business especially since we know they'll nag him to death, get a lien and who knows what else to collect.
The only "gotcha" is he should've had some reason to know the meter was wrong such as a sudden plunge in his bill about TEN years ago. Even then, it's been so long that could be explained by any number of things including rate changes, hours of operation fluctuations, expansion, remodeling, change in appliances, etc., etc.
He most certainly needs a lawyer to protect his rights because I can guarantee he'll get "screwed" by the city.
There would be no way of knowing until a signficant difference in usage was detected. That happened when the new meter was installed. Then it would be a matter of analyzing the reason for the disparity: which meter is more accurate? The old one or the new one? If the old one proved accurate, then the new one would be faulty and should be replaced. That apparently wasn't the case, though.