It's like what Zed0X said which I completely agree with, it is a mistake to try to help someone who did not ask for help, as in most cases, such humanitarian care doesn't always end well, as most end, you either end up making yourself an enemy to such friend, or the friend him or herself makes you his or her enemy, simply for trying to force him or her to leave what he or she enjoys doing.
So, before offering to help a friend, make sure such a friend have come to the point of self realization, and he or she have on his or own realized that his or her gambling activities aren't helping him or her, and have decided to stop but doesn't know how to, this is when it's clear that such a friends indeed needs help.
And if I have such a friends, I will do everything withing my power and knowledge to help him or her come out of his or her gambling addiction, things I could do includes advising, monitoring, discouraging him or her from gambling, even offering to be his bank account where he or she can save his or her money, so his or her money is safe from him or her using the money to gamble.