@very sad mom,
She shouldn't be disciplined. She needs education about personal boundaries, personal safety, and cell phone and internet safety.
Make this an opportunity for learning some life lessons not punishment for placing misguided trust in someone not worthy of her trust.
If you want some web links to help you with this, let us know. You won't be the first parent or young adult making use of them.
Be gentle with your daughter. This could be a life changing event for her, depending upon how it is handled.