Friends and families want help for loved ones. They have tried their best, and come seeking an intervention because their way hasn't worked. This is very painful process.
The intervention starts when the family and friends seek help. It is a time to learn new ways of coping, and it is a time to practice creating change. There are many resources to learn from and to use. For example, attending Al Anon is a great place to start.
I can make other specific recommendations and help in this intervention process if you wish.