Wicked Landlord

A landlord is threatening to evict a father and his beautiful young daughter, unless she agrees to marry him. There are a lot of witnesses and in a false gesture of sincerity, he offers her an opportunity to remain in the house without marrying him. He has a silk bag in which he says he has placed a white and a black stone from the footpath on which they're standing. If she picks the white stone from the bag, then she wins; if she picks the black, she loses. However, the young girl saw him place two black stones in the bag. She can't accuse him of cheating, because he would say that his good question was called into question and storm off without showing the bag. How does the clever girl win?