I know all about verbal contracts. The problem with verbal contracts is there is only oral evidence of the agreement. If the parties disagree about the terms, it's difficult to enforce an oral contract. In this case, his friend got half the winnings. Why would he sue? It's the IRS that would possibly question the arrangement and argue that there was no contract if it determined it was a tax dodge. Happens all the time.
IRS will most likely make him pay the entire tax because he received the money, a taxable event. He can then split it with his buddy, with whom he has a binding agreement.