It's not just gambling alone; what you said is true. When someone is excited, they can make huge promises to anyone. In this life, we should just learn not to put interest in the promises of someone if not we will get disappointed. What you can do is not say a word to him about the money, just let everything be and be his friend as usual.
If your friend had lost in his bet. You will notice he would stick to you for you to show compassion for him. Funny enough money won in gambling does not last forever, he will spend it all someday and then come back to you.
This is true, when many people are excited they tend to make promises that they ordinarily won't keep and when it's time to fulfill what they have promised, they will begin to have second thoughts. When some gamblers places bets, especially the ones that can give them huge wins, they will want to get emotional support from friends and in the process they will promise what they ordinarily can't do, so that is why I don't take promises made by gamblers on bets that they have not won because they will likely get greedy after the win and not fulfill it. But if they lose their bet they will expect you to comfort them, it goes to show that money shows the true character of people. You won't know the character of a person when he is broke, it is when he has a lot of money that the inner character will manifest.