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.
Money is an amplifier of ones nature, if one is the selfish type with money, when it comes to them, they will definitely going to be selfish when it comes. Gamblers shouldn't put in too much trust in what was promised to them especially if they are dealing with a gamblers. Humans act differently when we touch money, even the closest person you know can have a change of character when they make a certain amount of money. They said money is the only thing that can come between brothers. Which is true.
I cannot agree less on this. Money has made a lot of persons show their true character. They be hiding their true self's until they have the money, but one thing is certain in life, money doesn't come easy unless you cheat your way in getting it, but it leaves the hand faster than the way it came. Meanwhile in gambling i will not be surprised someone acts this way after winning a good sum of money. First thing first, do you know how long he has been toiling in betting sites before that win or how much he has loss in his entire gambling experience. For us not to think money is the cause of this let us stop putting our hope on money that was promised because money not in hand is no money at all.