Player leaves court during foul shot

[robertclasalle]

High school boys varsity: During the first shot of a two-shot foul by team A, B1 and the head coach are talking way in the backcourt (relative to where we're shooting) in front of B's bench. B6 is told by the coach to sub for B1, and then B6 reports to the scorer and is waiting there. While we're administering the first free throw, B1, knowing he's being subbed for, leaves the floor and heads toward a seat on the bench. Fans go nuts ("...he left the floor!!!...") There's lots of stuff in the rules/case books about coming ONTO the floor (unbeckoned), and leaving unauthorized during a LIVE ball, but there isn't anything I could find about leaving the floor during a dead ball. (And for our purposes, let's assume I haven't bounced the ball to the thrower for his first shot...but would it be different if I had?)