Was this an automated system he was dealing with? I can't imagine the type of person who would with a straight face demand a high school diploma after seeing a graduate degree.
Well, if you don't have any advanced degree, it's still pretty obnoxious to insist on documentation a few decades on, when maybe the high school or school district doesn't even exist any more. More painful and humiliating when a high school diploma is all you've got and you've been working for like 30+ years. I know a hospital that did this and remember an applicant loudly complaining.
A human insisted to his face that he provide a high school degree. As part of the hiring process. It must have been a hard requirement. Yeah, pretty strange. My Dad eventually got a copy of his diploma from the 70s and got the job.