What is due by way of supplication for a foreign girl who embraced Islam and then apostatized from it: Should one supplicate for her with a curse, or supplicate for her with guidance and forgiveness?

The first and best approach is to supplicate for the girl, asking for her guidance and her return to Allah and His religion. This was the guidance of the Prophet (peace be upon him) even with his enemies, as he prayed for tribes like Daws and Thaqif, saying: "O Allah, guide my people, for they do not know." The religion of Islam came as a mercy to the worlds, and the Prophet was sent as a lenient teacher and a bearer of glad tidings.