The date my friend has in his passport to next report his address is December 6, 2014. He's leaving Thailand for a week to a neighboring country on December 8th. So he is wondering - since you legally can do your 90-day report within 7 days after the "due date" - does he really need to report at all (other than 90 days from the day he returns to Thailand)?

Anyone know the answer? (if you're guessing the answer, please say so)

He has to report.

The 7 days is not an excuse for not reporting.

If he's been in Thailand for 90 days, he needs to report. It may not be noticed when he exits the country, but when he does report the next time they'll be able to see his exit stamp was after the date he was supposed to report. The 7 day allowance is only for making the report. It doesn't add 7 days to the 90 he was in country.

Agreed. He would have been in the country for 92 days. The week's leeway is if you are still here and basically covers weekends/holidays.