Intervening, or as some would put it Interfering, in someone else's life, is not something that most of us would do lightly. However, when that someone is a dear friend, a relative, or a loved one with a problem, many people feel duty bound to intervene.
It is a fraught situation, and a moral dilemma for some. But watching someone gradually destroying themselves with drugs or alcohol is not an option for caring people. The rights of an individual in those circumstances, I believe, need to be set aside to a degree in order to best help that person.
Intervention is not an easy process. It requires careful thought and planning, but it can be a very effective way to help someone face their addiction and get help. One of the best ways is to get a few people together who have been affected by an individual's addiction in one way or another, preferably in a neutral place, and talking to them, letting them know just how much their addiction has affected other peoples lives.
It's a wise move to involve a professional at an early stage. A person who is experienced in the field of addiction, and who can "referee" the intervention as it progresses through its various stages. A professional will not be emotionally involved and will help you as well as the addict while you deal with the situation.
There is no definitive correct way to intervene, a lot of the time you will be "winging it." you will probably be nervous, maybe hurt or angry to start with when you confront the addict about their problem.
You'll probably be scared they will hate you for bringing it up, and in fact this is a very common reaction. People often don't like being made to face their problems. But the idea is to let them know that their addiction affects not just them, but others too, and they need to know just how destructive that is.
Recovery for an addict, be it drugs or alcohol, is a slow, difficult process. However with intervention by friends and family it can be made less difficult by their support and caring. An addict often gets to the stage where they feel very alone and unloved. Loving support can make a huge difference to their eventual recovery.
People who have an addiction will usually be in denial, and say they do not have a problem at all. As someone who cares deeply for the sufferer it's important for you to make them realise that in fact they do have a problem, and it's affecting their friends and family as well as themselves. It's a difficult task, but they have to be made to realise that they need the help you are trying to give them. This will take a lot of willpower and persistence.