If I was the girl, I would not have pretended that I didn't see the cheating. I would ask him to show the two pebbles and I would say "why don't you just say that you want to marry me and there are only two options".
Then I would probably reject him, and work as hard as I could to raise the money and get my father out of jail. I didn't think of your smart idea! :)
hahaha! wow! your dad would have been thrown into jail ! Thanks for the honesty!