Substitution on foul shots

[drinkeii]

Player A1 subs in for A2 between the first and second foul shot of 2. The coach immediately sends A2 to the table before the official makes the ball available for the shooter's second shot, to sub for A3. The second shot is good, and the timer sounds the horn. The officials blow the whistle, pause a second or two and deny the sub. She ends up waiting until the next dead ball. Correct, or should she have been allowed to sub?