Dude... You went too easy on her.
I'd have the entire class put their electronics in the box at the door when they walk into class. They can get it back when they leave.
Regarding that student...
March her butt to the principle's office. You don't have the time for that crap. You tried once. Don't let it go if it happens again. It is not your job to parent her. But it is your job, as a mentor to guide her correctly. And at this point, consequences is where she needs to be guided. The consequences of her action in class would be, you get to go have a visit with the Principle.
Eventually she learns from it or she resists and spends all of her free time in detention.
In fact, you can, and you seem to have to, be tough on her. You can, let her know that it is for her own good and if she ever needs a safe place to discuss things, your door is always open, however if not you then seek the school counselor.
You go this!!!