how do I get her back
First off contact the cops. That's very serious. As previously stated he can be tried. He needs to be arrested for condoning it. Also, talk to her friends and let them know the severity of the situation. They may not be willing to talk to police, but will be to you. If its the opposite and you feel one of them knows something and isn't telling you, then notify the police of your notion and let them do their jobs. I hope you find her soon and she comes back safely.
You need to contact the police, and you can also contact the National Runaway Switchboard: 800-621-4000 (24 hours)
National Center for Missing and Exploited Children For reporting sightings of missing kids: 800-843-5678 (24 hours)
Contact your local police - she is a minor and he is an adult. He can be tried with kidnapping and if they have done anything sexual - he can be tried for that as well.
Do you know where shes at?