He had to have entered AS A PRO on Monday ( or whenever) since Master's cannot enter a pro-tier tournament.
He could have made the jump to Master with the first round he played, but would still be able to play the entire week .... with the stipulation that he can't use master-tier equipment. This happens all time. Any NEW tournament he enters would be at the master level.
As far as finishing 2nd, when you thought you had won? ... Read the tournament rules ( I know, that's a novel idea). There are several rules in place for tie-breakers ... can't have more than 1 winner. First it's the score over the last 6 holes, then the last 3, then it goes hole-by-hole. If it's still tied after all that, then the first to post the score is declared the winner.