what would you do?

[Self]

First is it legal? Technically it is not legal. Leaving the court during live ball is a technical. Would I call it in this situation, probably not.

Now as far as stopping play. I would probably have to see the play to say for sure. But from reading it, I would have probably waited for A to regain possession. If A left the court unaided then his team is going to have to play defense without him.

I say this but without being there having the feel of the game, seeing the player walk off, knowing the game situation. I possibly would have stopped it also immedialty if I felt the situation warrented it.