I don't know whether I think it's ethical or not overall, but it's at least a more optimal situation than if he had continued spending 8 hours a day updating spreadsheets by hand.
He's doing a better job than he was before, for the same price, and he gets more free time. Everyone's a winner. Admittedly, he is a winner by quite a bit more than they are, but he would have been perfectly within his rights to continue doing the work manually. Then they'd be paying the same price as they are paying now but getting work with more mistakes in it. Why would they want that?