Is informing a female friend that Christians revile God sufficient to establish proof of their disbelief, or is it necessary to present Qur’anic verses or prophetic traditions for that purpose?

The rule "Whoever does not declare a disbeliever to be a disbeliever is himself a disbeliever" applies to disbelievers whose disbelief is definitive, such as idolaters and People of the Book. As for a Muslim from whom something that is considered a nullifier of Islam has emanated, such as not declaring the People of the Book to be disbelievers, the rule does not apply to him, and it is not permissible for common people to judge him with disbelief. The judgment of disbelief upon a specific individual who outwardly manifests Islam is suspended until all conditions are met and all impediments are removed, and its determination rests with the Sharia judiciary. The questioner is advised to turn away from obsessive thoughts and consult doctors.