All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
Marrying a non-Muslim woman except from the People of the Book is forbidden.
For more benefit, please refer to Fatwa 82067.
It is unlawful for this lady to desert her husband or refuse to fulfill his rights just because he travelled for a long period. It is unlawful for her, and actually impermissible in the Sharee’ah, that she divorces him, as divorce is in the hand of the husband.
Ibn Maajah reported that the Prophet said: “Divorce is in the hand of the one who takes hold of the calf (i.e., the husband).”
But if the woman gets harmed by her husband, or if she can no longer live with him, then she has the right to ask for Khul' on the basis of what she and her husband agree upon.
On the other hand, this man should fear Allah and stay away from being in seclusion with non-Mahram women.
The Prophet said: “No man should ever stay in seclusion with a woman except in the presence of one of her Mahaarim (non-marriageable men).” [Al-Bukhari]
The Prophet also warned against entering a place where there are women only as he said: “Beware of entering upon women (i.e. a place where there are women only).” [Al-Bukhari and Muslim]
Since this man worships Allah and prays, then he is good. You should, therefore, admonish him, remind him of Allah and of the danger he faces by living with this non-Mahram lady. Tell him that if he fears Allah and abides by His Orders and avoids His prohibitions, Allah will give him a way out and ease his matters. Allah Says (what means): {And whoever fears Allah - He will make for him a way out.} [Quran 65:2]
Allah knows best.