First, she should pick one from the bag and let no one see the stone
Then, she have to throgh it a way, so nobady knows the picked stone.
However, the picked stone should be the oppeste, in color, to the stone that is in the bag.
So, the lender will have to show the stone in the bag.
and he cannot say that both stones are black because it will apper that he cheate. Yet, the girl will be saved becase she picked and through the white stone as the other people thinks.
Hope I got the right answer.