She IS an addict. Doesn't matter if she doesn't do it every day. She can't say no in a certain set of situations (when with certain friends). She is a habitual drug user.
I had (well, still have) a friend when we were the same age as your daughter. A social user for ages. She ended up being a daily user, whilst still living at home with her parents, much like your daughter really. She still worked, functioned etc. But she was killing herself and behaving really horribly. I had little to do with her for a while I couldn't tolerate her bullshit.
She only stopped and cleaned up her act when her parents threw out her vile boyfriend (who was abusive) and told her she either went to rehab which they would pay for once and once only or got out too, permanently, with no further support from them. Luckily they had the funds for rehab and paid for her to go, and she went. She was gone for four months and she didn't see a soul in that time. She got clean, blocked the boyfriend entirely (and he ended up back in prison in the end, for a long time!) and paid off her debts. She moved back in with her parents when she got out and gradually rebuilt her life, with their support, on the condition that she stayed clean. They drug tested her regularly for years.
Ten years later, she's happily married with two children and her own business. She's a light, occasional drinker but that's it. She's been clean every since and she no longer behaves in the awful ways that she did when she was using.
It can be done, but she says now that she'd be dead if it wasn't for her parents and although she absolutely hated them at the time she's hugely grateful to them for stepping in and saying 'no more'.