[greymule]

You do nothing except watch what happens next.

A throw getting away and hitting a former runner engaged in the game is nothing. It's not interference or even a blocked ball. (I am speaking as an ASA umpire who has had to study the NSA rules to cover certain tournaments.) Of course, intentional interference would be different.

Even if another fielder had a chance to record an out, I would not call interference unless I thought the former runner could have avoided the ball.