K.P.
You gave her a choice? Really? She's a child and you gave her tacit permission to leave your home. She does not have a pattern of making responsible choices and yet you gave her the choice to stay or go.
What do you do? At this point, you call social services and ask them. Legally, YOU are her parent until she turns 18. The friend's parents have no legal authority to supervise or support her. You call social services and get them involved. You also go to the home where she is right now and speak with those parents. Do they know what happened? Doubt it. Explain the situation in detail and see if they are still interested in temporarily fostering her b/c that's what they are doing. My guess is that they are going to tell her to leave. In which case, put her in the car and go home. If she runs away, you call the police.