Slim, that is truely terrible. As a parent of small children, I can relate to your friend's loss. I will keep her in my prayers, as well as her brother and parents.
That is so tragic and potentially preventable.
I used to be a lifeguard and teach swimming lessons when I was younger. I do not know what happened in this case, but when you are teaching these kids, you have to stay focused at ALL times. It is surprising how fast things can go wrong.
God Bless!
Doc