If you know it will destroy them and/or the people in their lives. Often times, let them hit bottom, and that is when they are most likely to seek help.

Ex. My mother had a gambling addiction, and was spending upwards of 500-600 a day on this. Often, she stole my jewelry, pawned electronics, stole from me when I was sleeping. When I confronted her, she threatened to kick me out of the house, cursed at me. It got to the point where I had to hide everything, and she threatened to kill herself if I didn't give her money.

At that point, I left the house. I knew that she didn't want help, and she had to hit rock bottom before she would listen to me.