Nothing, but the customer service person should have been asking, why did these people have a clean slate two months ago, and suddenly a 10 year old issue surfaces? The fact that they had a "new computer system" should have made them even more suspicious that something was wrong on their end.
I hope the CEO fires an unthinking customer service person and their supervisor, because this guy should not have had to go through this hassel.
The water company should credit the customer $ to compensate for his time lost trying to fight this thing.
I had verizon wireless do something similar to me. I got a new home phone when I moved to Florida. Somehow there were 6 cell phones attached to it. Each month they would send me a bill for the amount due for phones I never had. Went thru it every month, they’d correct and it would be back the next month. When the bill went over $9k I went to the state Public Utilities Commission. They finally corrected it and credited me about $1500.