I've had the misfortune of taking a few friends to rehab... It's never easy, no matter how you do it.
In my experience, it's only the ones who've really hit bottom hard that have any lasting success. More or less those that end up agreeing to go to rehab because they've run out of other options.
Often the best thing to do is the hardest thing to do when the afflicted person is someone close to you. Cut them off financially and emotionally till they're willing to get the help they need.
Alcoholics and addicts are a hard bunch to deal with when they're still using. Telling them that you're no longer willing to deal with them, until they get help, brings them a little closer to hitting bottom and limits the damage they can do till they're ready.