Marriage of a Muslim man to an apostate woman is invalid
14-8-2013 | IslamWeb
Question:
Assalam alaykum. There is a muslim lady who got married with a christian, and that lady became christian too, and they have 3 children. There is a muslim man who want to take her from that christian man and marry her in order to persuade her to come back to islam. If that lady acceptes to come back to islam and get married with that muslim man, so is that allowed in Islam? THANKS
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, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
Marriage of a Muslim woman to a non-Muslim man is invalid according to the consensus of Muslim scholars. Also, the marriage of a Muslim man to an apostate woman is invalid. Ibn Qudaamah 
said: “It is prohibited to marry an apostate woman, no matter what religion she adopts; because it is not proved for her the ruling of the people of the religion to which she converted by one's approval of her doing so. Based on this, it is not lawful for him to marry her with greater reason.” [Al-Mughni 7/131]
Accordingly, it is not permissible for that Muslim man to marry that apostate woman unless she repents, returns to Islam, separates from her non-Muslim husband, and her waiting period ends or it becomes clear that she is not pregnant from him. If that man hopes that she would return to Islam, he should strive hard to call her to Islam through some righteous Muslim women and not to handle that matter himself.
For more benefit, kindly refer to Fataawa 87672, 88997 and 85379.
Allaah Knows best.