First of all, Muslim youth develop very close friendships with their same-sex peers.
This "sisterhood" or "brotherhood" that develops when they are young continues throughout their lives, and serves as a network to become familiar with other families.
Allah, may He be exalted, says (interpretation of the meaning): “Whosoever desires honour, power and glory then to Allah belong all honour, power and glory (and one can get honour, power and glory only by obeying and worshipping Allah (Alone))” [Faatir ].
We advise this woman to end her relationship with that Christian man, because it is not permissible for a woman to form a relationship with a man who is a non-mahram to her.
"Dating" as it is currently practiced in much of the world does not exist among Muslims.
There are restrictions to whom a Muslim man can marry.
But pay the Unbelievers what they have spent (on their dower), and there will be no blame on you if ye marry them on payment of their dower to them. She could, in theory, leave the non-Muslim husband and marry a Muslim one.
But hold not to the guardianship of unbelieving women: ask for what ye have spent on their dowers, and let the (Unbelievers) ask for what they have spent (on the dowers of women who come over to you). If the non-Muslim husband does convert a new marriage is not needed. When there come to you believing women refugees, examine (and test) them: God knows best as to their Faith: if ye ascertain that they are Believers, then send them not back to the Unbelievers.
Praise be to Allah The Muslims are unanimously agreed that it is not permissible for a Muslim woman to marry a non-Muslim, whether he is Jewish, Christian or anything else, because Allah, may He be exalted, says (interpretation of the meaning): “And give not (your daughters) in marriage to Al-Mushrikoon till they believe (in Allah Alone) and verily, a believing slave is better than a (free) Mushrik (idolater, etc.), even though he pleases you.
Those (Al-Mushrikoon) invite you to the Fire, but Allah invites (you) to Paradise and Forgiveness by His Leave, and makes His Ayat (proofs, evidences, verses, lessons, signs, revelations, etc.) clear to mankind that they may remember” [al-Baqarah 1] “…then if you ascertain that they are true believers, 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 ].