Is it permissible for a Muslim man to marry a woman who was Muslim and then apostatized to Unitarian Christianity, and is she considered a woman of the Book in this case?

It is permissible for a Muslim man to marry women from the People of the Book, by the consensus of the scholars, based on the Almighty's saying: "And the chaste women from among those who were given the Scripture before you." However, if a woman embraces Islam and then apostatizes, she must be asked to repent. If she repents and returns to Islam, then that is good and excellent. Otherwise, she is an apostate, and the rulings of apostasy apply to her, and it is not permissible for a Muslim man to marry her, by consensus. She should be advised and have any doubts removed from her, as she might be guided.