Time Out

[Gimpay]

Team A is up by one point with 20 seconds left in the game. Team A has the ball in their frontcourt and B1 steals the ball. B1 and B2 go on a two on one fastbreak and player B1 loses the ball and ends up getting trapped in the corner on his own baseline and then passes to B2 who is standing by the basket for a layup. As player B2 goes up for the uncontested layup and is in the air the referee blows his whistle and points states that the coach of Team B has called a timeout. The coach for team B insists that he did not call a timeout because they had an extreme advantage. The referee continues and charges the coach with a timeout and waves off the basket made by player B2 who was in the act of shooting. What is the rule when a timeout is called and a player was already in the act of shooting as B2 was doing?