Most gamblers hide their gambling is because they feel ashamed or afraid of being criticized by their family. So if you are thinking of telling your friend's addiction to his family, try to consider his or your relationship with his family. See if telling his family would have a good impact on his gambling addiction. Family can help better with addiction, they can support their loved ones well. And if not, then it's better to support your friend and push or recommend him to seek professional help.
I will reward a merit for a fresh idea. A lot of people here say that "yeah, family will help", "family will never set down" and etc, but a lot of them forget that relationships with family can be easily broken, and family can do most of pain when help is needed.
Btw it is funny how people say here that family can help a gambler, and parallel they support idea of keeping gambling habit as a secret
Such a funny double standard situation. Such a selfish people, that refer to family only when they are in trouble, but not when they are victories with lots of money