Interesting scenario! I don't think the rules specifically cover this situation. Anyone who answers is going to have to pick a few rules and apply them.
Here's my take.
Anytime after the ball entered the BC and while the ball was still live, the BC violation could have been called. Once the basket is scored and the ball becomes dead, it's too late to call it. It's not a correctable error situation as defined in 2-10. It's no different than trying to call traveling or 3 seconds after the shot has already been made. My solution would have been to count the basket and give the ball back to A1.
And, be sure to point him in the right direction.
