without a written agreement for success fee, how did Richard Suen win the suit against the casino?

Without the written agreement about the success fee, it is Richard Suen’s words against the casino’s words. How did Suen manage to win suit? I like to learn from Suen becos I wish to sue someone who promised to pay me my service fee but dishonour it after job done, and that someone refused to sign the Fees Agreement for me.