To the people that knew about the bet and witnessed its results, no, the bet did no good and did not prove anything. The reader can see that the lawyer grew in a moral sense. He was able to achieve a sort of peace with himself and the world. He no longer is motivated by materialistic pleasures and pursuits. He learned a lot of human nature and man kind itself. His knowledge grew a lot because of all of the books he was reading. That all sounds great, but the experience left him disgusted with mankind too. And that's where he has to live now that he's out of prison.
The banker didn't wind up much better. He's next to broke and now realizes that he was prepared to murder an innocent man in order to keep his material possessions. Of course nobody knows that though. To everybody else, the bet proved that the banker was right. The lawyer is a shell of who he used to be and the banker gets to keep his money. Nobody will ever know why the lawyer left early, because the banker hid the note. I feel that the only thing the bet did accomplish was to destroy two men.