What is the religious ruling regarding the action of a 27-year-old Muslim daughter who lives with a Christian man in the same house, is pregnant by him through Zina (unlawful sexual intercourse), after separating from her Muslim husband whom she married without her parents' consent, and how should her parents deal with her?

Regarding her marriage, you mentioned that she separated from the one she married. It appears she separated from him by divorce, otherwise, their marriage would have been invalid because it was without her guardian's permission. As for her relationship with that Christian man and the fact that she became pregnant through fornication with him, what she did is a grave crime against her Lord, her family, and herself. She must hasten to repent. As for her family, they must enjoin good upon her and forbid evil. If they can separate her from him by force without causing a greater harm, then that is obligatory. They should not tire of advising her and supplicating for her guidance. We would like to point out two things: First, what happened is a negative consequence of residing among polytheists. Second, this may be due to shortcomings in raising her according to Islamic morals and manners. Consequently, the parents may bear a share of sin if they were negligent in her upbringing.