What was the action in the hand? Did he just go all-in out of no where, did he check-raise, or what? What was his position in the hand? I can't really imagine any remotely intelligent player flopping the absolute nuts with virtually no chance of getting outdrawn and just moving all-in, so it's probably more likely he/she is bluffing (perhaps with the ace), or protecting a vulnerable hand (a weak flush with a low or middle suited connector). I'd think that flopping the ace high flush would be an automatic slow-play.

Anyways, this is a pretty easy call with the second nuts.