Joel,
According to the USGA, the odds of a 9 handicap beating his handicap by 7 in a tournament is 1200:1. So, while it can be done, it is very easy to tell if someone is "careering" it, or shooting a low stress 74. Knowing Mac, I suspect the player was the latter.
And yes, I know it was not a tournament, but I couldn't find non tourney stats and I figured he pressure of playing with Mac closely approximates major tournament golf.