First, it needs to begin with the person itself, no one can help him if he or she's not willing to accept that he possess addiction, once he realize and admits that then aid will be more effective, just like what you mentioned allowing to go to rehab that's something that will give him a full stop into gambling, with his willingness to cure himself everything will follow, unless he choose to gamble again that will start the flame and give that green sign to start ruining his life again, it's a tough advise as it will pass in the ears without any work after the advise then nothing will change.
The first thing that he can do is accept what happens to him and trying to figure out how he can fix that matters. That will makes people who wants to helps him can start to looks for the ways out from the problem. If he addicted to gambling, he must trying to solve this and with the helps from people around him, he can gets the way to cure his addiction.
Without the acceptance that he had addicted to gambling, he will not listen to other people that suggests him to cure his gambling addiction. Other people will difficult to asks him to go to rehabilitation center because he will refuse to go that place. He will consider that everything still fine and he can control himself but it is not. He will only gets deeper in gambling without have a chance to solve his problem.