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.