There isn't quite enough information here to figure out whether he busted the regs or not.
You could go over 16 hours of duty time if it happened due to an unscheduled delay, as an example, you block out and your flight is scheduled to block in just over 15 hours, but, due to circumstances beyond your control (adverse weather) you get delayed beyond that, the Feds aren't going to hammer you the same way they would if you take off at the 15 hour point for a scheduled 1+01 flight.

And don't confuse duty time with block time. 8 hours of block time in a rolling 24 hour clock (looks backwards and forwards) imposes various amounts of rest.

There is a 16 hour duty day that your bud may or may not have busted. (121.505 (b) No pilot of an airplane that has a crew of two pilots may be on duty for more than 16 hours during any 24 consecutive hours.)

If the DH was a company scheduled event, then I would run/not walk to file a NASA ASR report discussing the issue. http://asrs.arc.nasa.gov/

A company scheduled DH does not count as block time, but does count as duty time. Which also means that it does not count as Rest time.

If he was released from duty, to a legal rest period, with a scheduled DH home the next day.....and he elected to just suck it up and go home.....then, he's okay......unless the stretch to a 16:30 duty day was avoidable