I can share my experience with interventions. A couple family members and I read up on interventions and we had a professorial facilitator lined up. We set a date and scoped out a couple centers and did the insurance homework. A week prior to the planned date Mel decided to check in herself voluntarily.
She relapsed a shortly after and voluntarily checked into a second rehab several months later, then relapsed shortly after completing that program. After we were divorced she voluntarily checked into a third program and relapsed again a few months after completing that program. She passed away in Sept of 09 from cirrhosis.
I'm a recovered cocaine addict and have been clean for 28 years. What I learned from my experience with addiction is a much more in your face basic reality than what you learned from your experience. Some people get clean and survive, some people die. An intervention would not have saved Mels life.
If you plan an intervention I would read up on it and seek out professional help. Good luck.