Cheating on Tests

I have a friend that supposedly does very well on her exams and quizzes. She was a like a "child prodigy" in my eyes because of her excellent grades. Apparently, though, I have to choose my friends more wisely.

I don't have any classes with her this year except this one class. However, outside of this class, I don't know anything about her... so imagine my surprise when one of my close friends tells me that she's a fake.

Apparently, she cheats on all her tests and quizzes. When 'bring your tech' was introduced to our school, she would secretly search answers to her test/quiz questions on her device. When there was no technology allowed, she'd take notes and hide them somewhere and cheat off from there. Obviously, she doesn't deserve these 'excellent' grades.

I don't know what to think. It's very shocking to me, because I was a pretty close friend of hers. She'd hug me, make my day happy, etc., when she'd be a completely other person on the inside. The positive thing to this situation is that other people know. Don't get me wrong now, I'm not one to immediately 'hate' her because one person told me. I asked a bunch of other people to see if they also knew - and they all said they 'hate her guts', 'she's fake' , etc.

Can you please tell me what I should do? I don't really want to tell the teacher/authority because she can make my life at school horrible if she wants to - especially if I ratted her out. Plus, she has a lot of fake friends that really don't like her but still pretend like they do... so should I "act" like her friend? Please understand that I am really confused as to if I should like her or not.