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.
Then why do we have promises, as something sacred? Or is it not because addicted people use it to gain? Those people (addicted) say them and create anticipation and hope for all the others, only for their personal gain. Because they are sure that "promising" can persuade others. That doesn't mean that promises are words of the air.
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.
For sure he will. The question though is do you seriously want to be the financial (and not only) crutch of an addicted person who only remembers you when he loses and wants to satisfy his addictive need? Sorry, but I don't want and don't consider such people my friends.