She wants to marry a man whom she loves but he previously committed zina with her mother
Question

                I have a friend who wants to marry a man whom she loves dearly, but the problem is that this man once committed zina with her mother who was married and had three children at that time. It was a mistake committed in a moment of heedlessness. My question is: what should she do? Is it permissible for them to marry?.
Praise be to Allah.
      The fuqaha’ differed concerning one who commits zina with a 
      woman: is it permissible for him to marry her daughter or not? There are 
      two views: 

    The Hanafis and Hanbalis are of the view that it is not 
    permissible for him; they regarded haraam intercourse as being like halaal 
    intercourse with regard to who becomes a mahram as the result of a 
    marriage. 

    The Maalikis and Shaafa‘is are of the view that he may marry 
    her. 

    Ibn Qudaamah (may Allah have mercy on him) said: Haraam 
    intercourse creates the same mahram relationships as halaal intercourse or 
    intercourse as the result of a dubious marriage - meaning that those who 
    become mahrams as a result are confirmed in all three cases. So if a man 
    commits zina with a woman, she becomes haraam to his father and son, and her 
    mother and daughter become haraam to him. Something similar to that was 
    narrated from ‘Imraan ibn Husayn, and this was also the view of al-Hasan, 
    ‘Ata’, Tawoos, Mujaahid, al-Sha‘bi, al-Nakha‘i, al-Thawri, Ishaaq and ashaab 
    al-ra’y. 

    Ibn ‘Abbaas narrated that haraam intercourse does not make 
    anyone a mahram. This was also the view of Sa‘eed ibn al-Musayyab, Yahya ibn 
    Ya‘mir, ‘Urwah, al-Zuhri, Maalik, al-Shaafa‘i, Abu Thawr and Ibn al-Mundhir. 
    End quote from al-Mughni, 7/90. 

    The more correct view is that of the Maalikis and Shaafa‘is, 
    that it does not make anyone a mahram. See also the answer to question 
    number 78597. 

    But our advice is that this marriage should not go ahead for 
    two reasons: 

    1.

    So as to be on the safe side and avoid an area of scholarly 
    dispute, because she is haraam to him according to many scholars, especially 
    the Hanafi madhhab that is prevalent in your country. 

    2.

    If he marries her, that will lead to him being close to her 
    mother and being in touch with her, and there is the fear that this would 
    remind him of that haraam action or tempt him to do it again, or it may be a 
    cause of the woman (the mother) being tempted. We ask Allah to keep us all 
    safe and sound. 

    If your friend is in a relationship with this man, this 
    indicates that he is corrupt and deviant, as he did not have any religious 
    commitment that could have deterred him from committing zina with a woman 
    who had three children and forming a relationship with her daughter. Such a 
    man cannot be trusted at all. Your friend has to end her relationship with 
    him. Perhaps Allah will send her a chaste husband who is not contaminated 
    with this evil and sin. 

    And Allah knows best.