He called and told them they'll need to find coverage for that day, as it wasn't a request, but him notifying them he will not be available that day. If asked he plans to tell them that shift isn't his responsibility to find a replacement.
He's not full time, and it's unpaid time off so it's not messing with payroll or anything like that.
He even took a photo of the page in the time request off book, because they did the same thing last year and “didn't have the page anymore, so we don't have the ability to prove he did or didn't request it off.”
Cover your asses, people. Even if you're unsure if it matters, the day you need proof is the day you don't have it.