Some people recommend that you try to get your friend to go to a gamblers anonymous meeting, but I do not recommend this. Gamblers Anonymous is a great organization that truly helps those with gambling problems stay away from gambling, but before you are ready for this most people need one on one sessions with a psychologist or a counselor trained to deal with gambling addiction.
After all is said and done, you must be prepared because most of the time the gambler continues to gamble, maybe they will make a brief attempt to stop before starting again in secret.
With most addictions the person with the problem needs to hit what they call a "bottom". This is when the gambler has lost everything that really meant anything to them, such as family and friends, and it is only at this point many will see the problem and seek help. If you know someone who is unwilling to admit their gambling problem I recommend that after you have tried your best to get this person to seek help if they do not, you may have to be prepared to leave them, remember at some point you have to abandon a sinking ship before you go down as well.