He wants to marry a Christian girl with whom he committed Zina
30-1-2012 | IslamWeb
Question:
Assalaamu alaikum Shaykh', One of my muslim friend wants to marry a christian girl with whom he has cohibited.He is the only one who has cohibited with her. He has realised his mistakes and is repenting. In the Quran its permitted to marry chaste women from the people of the book. Please correct me ya Shaykh.Is he allowed to marry her?
Answer:
All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad 
is His slave and Messenger.
Marrying a woman from the People of the Book is permissible but with the condition that she is chaste; Allaah The Almighty Says (what means): {This day [all] good foods have been made lawful, and the food of those who were given the Scripture is lawful for you and your food is lawful for them. And [lawful in marriage are] chaste women from among the believers and chaste women from among those who were given the Scripture before you.}[Quran 5:5] Ash-Sha’bi 
said (while interpreting the above verse), “The chastity of a free non-Muslim woman who lives under Muslim protection (rule) is that she does not commit Zina (fornication or adultery)…”
Ibn ‘Atiyyah quoted this from him in his Tafseer (exegesis of the Quran). This means that if she commits Zina voluntarily, then she is not considered to be chaste.
Therefore, it is not permissible for the man whom you mentioned in the question to marry that girl unless she gives up and regrets what she did, becomes chaste and guards her private parts from illegal sexual acts.
On the other hand, the scholars 
stated that a Muslim should not marry a woman from the People of the Book even if she is chaste as this leads to many disadvantages.
For more benefit on the disadvantages of marrying a woman from the People of the Book, please refer to Fatwa 84159.
Hence, he should be keen on marrying a pious Muslim girl hoping that Allaah The Almighty will bless him with pious children whom she would nurture on the creed and morals of Islam.
Allaah Knows best.