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.
Well, yes, actually it doesn't matter even if his friend loses money from his winnings because he acted too rashly by implementing his greed, but for the promise issue, that's another thing, the point is that he has managed to win regardless even though in the end he has to lose the money again, and also OP is not involved at all with his friend's greed, or what I mean is that his friend made his own decision to implement his greed, meaning that maybe his friend is someone who is not responsible for his promise, he should at least apologize to OP if he doesn't have the money to keep his promise due to his greed. And if there is no apology whatsoever from that person, then maybe this can be used as a lesson for OP not to put too much hope in other people, as you said that not everyone can be trusted.