That was big of him and I'd bet, unexpected. In reality, he shouldn't mind, being that you're doing some testing FOR him.I really pissed off a local station several years ago when I tested the fuel at the pump. Tried to explain that it was a customer requirement of mine and I needed to just verify the fuel before putting it in the customer's equipment as I had gotten E10 fuel that was mistaken pump into the fuel tanks before. He was smoking hot.
A week later he personally came to my shop and apologized as he had contact his distributor and they said it is possible for the delivery driver to make a mistake as they carry both fuel types on the same tanker.
He most likely has no idea what is being put in his tanks. It's ,as was said, the delivery driver that does the delivery, and even he may not be sure what he's hauling. Only real way to tell is to test the fuel!