Praise be to Allah.
We praise Allah, may He be exalted, and thank Him for guiding this girl to enter Islam, and we ask Him to make her steadfast in adhering to truth and guidance, and to increase her in guidance and steadfastness.
Marriage of a Muslim woman to a kaafir man is undoubtedly haraam and it is an invalid contract, but it is not kufr if a Muslim woman does that and marries a kaafir man, even though it is haraam for her. Allah says (interpretation of the meaning): “then if you ascertain that they [Muslim women] are true believers send them not back to the disbelievers” [al-Mumtahanah 60:10].
Ibn Katheer (may Allah have mercy upon him) said:
This verse is the one that made it haraam for Muslim women to marry mushrik men. It was permissible at the beginning of Islam for a mushrik to marry a believing woman. Hence Abu’l-‘Aas ibn al-Rabee‘ married the daughter of the Prophet (blessings and peace of Allah be upon him) Zaynab (may Allah be pleased with her), when she was a Muslim and he was a follower of his people’s religion.
Tafseer Ibn Katheer, 8/93
In al-Mawsoo‘ah al-Fiqhiyyah (7/133) it says:
It is not permissible for a Muslim woman to marry a non-Muslim man, even if he is a dhimmi or a kitaabi (one of the people of the Book, i.e. a Jew or a Christian). This is according to the consensus of the fuqaha’, because Allah says (interpretation of the meaning): “And give not (your daughters) in marriage to Al‑Mushrikoon till they believe (in Allaah Alone)” [al-Baqarah 2:221] and “send them not back to the disbelievers. They are not lawful (wives) for the disbelievers nor are the disbelievers lawful (husbands) for them” [al-Mumtahanah 60:10].
If the family of this girl insist on marrying her to that kaafir, we think that she should state openly that she is a Muslim and seek help with one of the trustworthy Islamic centres or a trustworthy Muslim family, until she finds a Muslim husband to marry her, then she can live with him. The timing of showing her Islam openly and leaving her family, whichever comes first, depends on her evaluation of her own circumstances. If she is afraid that if her family find out about her being Muslim they will detain her or prevent her from running away for the sake of her religion, or they will force her to go back to kufr or to marry a kaafir, then in this case we advise her to leave them first and go to another place where she will be safe, either under the protection of the government in her country if they offer protection in such cases, or under the care of one of the Islamic centres in her country.
Muslims are excused for staying among the kuffaar and not practising their religion openly if they are weak and oppressed among them and cannot leave and flee from them. But the one who finds a way out no longer has this excuse, so it is not permissible for him to stay among the kuffaar, not in their house or in their country. If he can find a safe refuge in the same country, he should move there from his house, but if the country itself is not safe, he should move from his house to another country where his life will be safe and he will be able to practice his religion openly.
With regard to mere fear about the family's reputation, this is not a valid excuse unless her religious commitment and her life are in danger because of it.
Who knows? Perhaps showing her religion openly will bring a great deal of good to her and her family, and some of her family members may become Muslim. This is what has happened with other sisters who showed their Islam openly. And Allah says (interpretation of the meaning): “it may be that you dislike a thing and Allaah brings through it a great deal of good” [al-Nisa’ 4:19].
Whatever the case, we know that the situation is difficult for this girl, but her religion requires her to do whatever she can in order to adhere to it and practice it openly, and not fall into haraam.
We ask Allah to relieve her distress and take away the calamity and to guide her family to Islam. And we ask Him, may He be exalted, to bless her with a righteous husband and good offspring.
We should point out a number of things:
If a Muslim man marries her, it is essential for her to have a wali (guardian) who will do the marriage contract for her, because there is no wilaayah (guardianship) for a kaafir - even if he is a kitaabi (Jew or Christian) - over a Muslim woman. If there is a Muslim man among her family, then he is her wali. If there is no Muslim man among her family, then the Muslim qaadi (judge) or mufti or official in charge of Muslim affairs, or the director of the Islamic centre to which she goes or gets married in, may act as her wali.
We should point out to you that it is not permissible for you to have a relationship with this girl, as she is a stranger (non-mahram) to you. If you want to help her - which is obligatory for you if you are able to - then help her to find some Muslim sisters who can take care of her, or a trustworthy Muslim family with whom she could live, or offer her your help and advice through your wife, if you are married, or through your sister or mother.
Do not remain in a relationship with her, because you are a stranger (non-mahram) to her, and Islam forbids such relationships.
And Allah knows best.