If your friend got caught up in a schedule change and missed the flight, I have to assume either it was by a few minutes and he was just runnig too late, or it was a major change of more than an hour or two. If it was more than an hour, NW really should have tried to notify him, but then again, many people are hard to find.

Personally, I would never head out for a flight that I had not double checked my intinerary and seat assignment within days of the flight, often hours.