The play was perfectly legal. The coach was wrong. Big surprise.
A player may bounce the ball OOB prior to making the throw-in pass. What's the difference between purposely bouncing it and dropping it to the floor accidentally? In this case nothing.
9.2.2 SITUATION D: A1 dribbles the ball on floor on the out-of-bounds area before making a throw-in. RULING: Legal, a player may bounce the ball on the out-of-bounds area prior to making a throw-in.