15 year old refuses to return to custodial parent...help?

15 year old neice is visiting father out of state for scheduled visitation. She comes from Texas, and is visiting Illinois, she flys, she refused to get on the plane to return home. She cried, and her father strongly encouraged her to get on the plane, and pulled her towards the security gates, once there she continued to state she was not going back, and wanted to stay, and continued to refuse to get on the plane, security said they couldn't let her thru if she didn't not want to board, the airlines refused to let her on, unles she was willing, the plane was delayed 20 minutes while her father encouraged her to get on the plane and they would work this out later, she could not miss her flight. She still refused, another flight has been scheduled for her to return ASAP, but she is stating she will not get on that plane either, the mother has stated if she doesn't get on the plane and return home, she will drive and get her. What will happen if she refuses to get in the car with her, throws a fit whatever, and will not get in the car. The parents have joint custody, and the child lives with the mother. Help please