Hi Ctobin,
That player was an Amateur when they entered the event and earned their promotion to Pro a little bit later. They are allowed to remain competing in the Amateur tier for the duration of that particular tournament and any other tournaments they entered while still a qualified Amateur. To keep it fair to the rest of the Amateurs that player will only be allowed to use Amateur or Hack tier equipment if they want to post any more rounds for that tournament.
That player will compete in the Pro tier for any subsequent events they enter since turning Pro.