Throw In NFHS game

[Stripes33]

After a made basket A1 takes the ball out of bounds and has his feet half way in bounds and half way out of bounds. A1 throws ball to A2 in bounds, what's the call? I called a violation immediately.

I ask because after the game a very experienced official stated that the proper mechanic would be to continue the 5 second count and call the violation then. I can understand his thinking if A1 was still completely in bounds when he threw the ball to A2 but that was not the case. My response to his scenario is what happens if B1 steals the ball and scores or if A1 throws a long baseball pass for a layup at the other end before the 5 second violation.

Thoughts?