And in Islam, it is acceptable to lie to “infidels” to advance Islamic goals.
What they would probably do is say that she was a "confused" teenager and "imprison her for life" in her parents' home, exhorted every day to return to Islam and deny Christianity. Imams would pray for her to "return to her senses". She would not be allowed to go to church or to hang out with her friends. She would be in prison.
I get the feeling that with this girl's deep commitment to Christianity, eventually she would publicly rebel and state that she will never return to Islam and she will forever follow Christ. Then someone would have to kill her. It might not be her father; whoever did it would be viewed as a hero in the Islamic community. They can't allow a happy apostate to exist. It sets an unacceptable example for others. After she's dead, CAIR and the other Islamic apologists would strongly protest such an un-Islamic murder. You know the drill. Other Muslims who might be considering converting would be scared to death.