I'd be more than a bit careful, if she's anything like my daughter was when she was 18 then talking to her will make her worse, just to show she can and will do it her way. Doesn't matter how nicely you do this, I'm pretty sure it will backfire on you.
My husband had a quiet talk with our daughter over her clothes, it made her ten times worse.....as I guessed it would. Until she grew out of it herself, and luckily there was no mobile phones or social media then.