It's extremely difficult to take action and help someone who does not want it. I am not implying all these people knew and did nothing about it. No one wants to lose their friend or precious daughter. That is the part of addiction that is a disease. Someone who does not want to be sober will not succeed at rehab. She was also legally an adult, so her parents could not forcibly put her in rehab.
I've known people who have done extremely well at rehab, only had to stay for 90 days and can get back to living their lives with some NA meetings to keep them on track. I know people who have gone to rehab at least 10 times, have been in and out of halfway houses and they want to get better but they can't seem to stay on track. And unfortunately I knew people who went to rehab, came out and took the dosage of heroin they were shooting up prior to rehab and died instantly. Relapse is also no joke. But until you make up your mind that you want to get better no amount of help, interventions, or rehab will help you. That is the reality of drug addiction.