That is interesting... usually there is a 3 percentile penalty for missing a question - your friend was very lucky and must have gotten it as an experimental question or perhaps he marked the question but did not hit submit? (that still counts).
If he answered every question perfectly, he still should have gotten only 97th percentile, which is 50, not 51 per this table: gmat-scores-83890.html
So.... there are 3 explanations:
1) This was an experimental question and did not count though missing even that question should still count, so probably not.
2) He marked the question but did not hit submit (most likely)
3) 97th percentile is actually 51 but GMAC did not publish and updated table (? Possible)
I asked my friend what exactly happened.
He is 100% confident he NEVER clicked submit and section timed out. But he doesn't remember if he selected any option before the section timed out. Maybe he did select but he said normally he used to select the answer and hit submit right away, so chances of him selecting an option is low. Anyways, I don't think we should take the marking the last question to the last second. I will target for 5-10 sec before the last question if I am running out of time.