When I was at UoM a few years ago, the advertised start was writing time, and you'd be let into the exam room ~15 minutes early or whatever for reading time. That was also back when you only had two exams per day, 3 hour exams at 9.30 and 2 hour exams in the afternoon. There are some old statute documents on the website with the same information but I'm not sure how old they are exactly.
Regardless, I can't see how DU has a problem if nobody told him so at the time. If the invigilators thought he wasn't allowed to write, they would have told him to stop (I've seen that happen once or twice, they just tell the offender to put his/her pen down). Knowingly letting him continue breaking the rules without telling him to stop would be crazy imo, I've never seen it happen.