What should a girl do after discovering that her close friend has converted to Christianity, and she hasn't received any help from the authorities she contacted; and would reporting her be considered sectarian strife?

The most severe dangers facing Muslims are those that threaten their faith, which is represented by the enemies' attempt to turn Muslims away from their religion by force or deception, as Almighty Allah says: ﴿And they will continue to fight you until they turn you back from your religion if they are able.﴾ Muslims have been subjected to intellectual and missionary invasions aimed at uprooting Islam from their souls. The Muslim's duty is to preserve his religion and resist apostasy in all its forms, by enjoining good and forbidding evil, inviting to Allah with wisdom, striving to resist missionary campaigns, and warning friends and relatives against them.