There is nothing you need to do to talk to him just keep quiet and see if he will keep his promise and it should go without saying, if he has the mind and considers you as his friend he will give what has been promised before. Indeed sometimes victory can make someone forget and they act greedy to enjoy their own victory without thinking about his words clearly he cannot be trusted in anything as a friend you just act know enough and do not have to deal with a friend who has such a nature.
It is true that sometimes winning can change your mindset and I have seen this in many people. You can be tempted to bet more with the money you have earned through gambling and the tendency to pass off the success you have achieved through others as your own success. In fact, the op did not show a tendency to be greedy with his friend's winnings and this shows his personality and it is positive that he does not get involved in other people's money for his own benefit. I think he will not defend his opponent because he is showing his selfishness between betting and winning. This is a reminder and as op said that most gamblers have a change of mindset after winning a bet and it is safe to say that many gamblers have the same tendency. Also, by being smart and knowing those friends, you can make yourself more tempted by avoiding them from your friend list.