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.htmlSo.... 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)