How can a girl, whose Muslim father and Christian mother are separated, and who wishes to convert to Christianity, be convinced to return to Islam?

To bring your friend back to Islam, you must be sincere in advising her, show her love, affection, and honest words, and offer sincere prayers to Allah for her salvation. Be sure to engage in dialogue with her and plant questions in her mind about why she apostatized, what she lacked in Islam, and what she found in Christianity.

Focus on two great principles: 1. The greatness of the Oneness of Allah (Tawhid): This is the foundation of the religion of Islam and of all the messages of the prophets, that there is no god worthy of worship except Allah, as stated in Surat Al-Imran (64-71). 2. Belief in all of Allah's messengers and loving them: Islam is the only religion that believes in all messengers and prophets, and acknowledges their rights and status. Whoever disbelieves in Muhammad, peace be upon him, has disbelieved. Allah Almighty said: "The Messenger has believed in what was revealed to him from his Lord, and [so have] the believers. All of them have believed in Allah and His angels and His books and His messengers, [saying], "We make no distinction between any of His messengers." And they say, "We hear and we obey. [We seek] Your forgiveness, our Lord, and to You is the [final] destination."" Al-Baqarah/285.

Finally, pray for her in her absence that Allah guides her, for prayer is a great weapon.