You are joking, right?

Did they open the cooler and determine the mode of failure?

They performed something called a"leak down" test. (I wasn't there.)

Demand that he provide documentation from the manufacturer
to indicate that the area of zinc remaining was below that which
protection is deemed "adequate."

Here's where it gets even more interesting.
This is a replacement aftercooler. The company I bought it from in Seattle
thinks that it doesn't even require a zinc, and that the fitting is simply a
drain plug.