You'll hear many different versions of advice about this, because individual experiences vary so much.
In general, IMO, only three things can be said with certainty:
1) You can only help by being a friend, but not an enabler. Be true, be steadfast, be fair, be firm.
2) You cannot act for him, he must make his own choices.
3) You must understand that not everyone can or will overcome this kind of stuff, no matter what. Don't let that make you decide to give up, but realize you're only responsible for what *you* control.
How we have made progress in dealing with alcohol abuse in our society is IMO an amazing success story when compared to the way we've failed regarding other intoxicants. Its too bad your friend is suffering in part due to that.
culero