If he marries a woman she becomes permanently forbidden in marriage to his father
Question

                Is the wife of a son who has died, with whom the marriage was not consummated, regarded as permanently forbidden in marriage, or is she only temporarily forbidden?.
Praise be to Allah.
    If a man marries a woman, then simply by virtue of the 
    marriage contract she becomes permanently forbidden in marriage to his 
    father, even if the marriage was not consummated, whether he dies or he 
    divorces her. 

    Allaah says, when mentioning the women who are forbidden in 
    marriage (interpretation of the meaning): 

    “the wives of your sons who (spring) from your own loins”

    [al-Nisa’ 4:23]

    Ibn Qudaamah said in al-Mughni (9/524): 
When a man has concluded 
    the marriage contract with a woman, she becomes forbidden in marriage to his 
    father simply by virtue of the marriage contract, because Allaah says 
    (interpretation of the meaning): 

    “the wives of your sons who (spring) from your own loins”

    [al-Nisa’ 4:23]
This refers to the wives 
    of his sons. And there is no scholarly difference of opinion on this point, 
    praise be to Allaah. 

    Ibn al-‘Arabi said in Ahkaam al-Qur’aan: 

    Every woman who becomes permissible (through marriage) to the 
    son becomes forbidden to the father permanently. 

    Imam al-Shaafa’i said in al-Umm: 

    Allaah says (interpretation of the meaning): 

    “the wives of your sons who (spring) from your own loins”

    [al-Nisa’ 4:23]

    So any woman whom a man marries becomes forbidden to his 
    father, whether the son consummated the marriage with her or not. Similarly 
    she becomes forbidden for his grandfathers on both his father’s and mother’s 
    sides. 

    The Standing Committee for Issuing Fatwas was asked: Is it 
    permissible for a father to marry the woman whom his son divorced, if he did 
    not consummate the marriage with her? 

    They replied: 

    If the son concluded a marriage contract with a woman, then 
    she becomes permanently forbidden to his father and grandfather, including 
    fathers and grandfathers through both descent and breastfeeding (radaa’ah), 
    even if he did not consummate the marriage with her and was not alone with 
    her. This is indicated by the general meaning of the verse in which Allaah 
    mentions the women to whom marriage is forbidden (interpretation of the 
    meaning): 

    “the wives of your sons who (spring) from your own loins”

    [al-Nisa’ 4:23]

    The Standing Committee was also asked (18/210): 

    What is your opinion concerning a man who married a woman 
    then divorced her, and her ‘iddah ended – is this women permissible in 
    marriage for the man’s maternal grandfather or is she forbidden to him? If 
    she is forbidden, what is the evidence for that, may Allaah reward you? 

    They replied: 

    It is not permissible for a man to marry a woman with whom 
    his son or his son’s son or daughter’s son, whether through descent or 
    breastfeeding, has concluded a marriage contract. That is because Allaah 
    says, when mentioning the women who are forbidden in marriage 
    (interpretation of the meaning): 

    “the wives of your sons who (spring) from your own loins”

    [al-Nisa’ 4:23]

    So if a man divorces his wife or dies, she is not permissible 
    for his father or his grandfather, whether on his father’s or mother’s side, because this ruling applies equally to both paternal and maternal grandfathers, as the verse is general in meaning. 

    And Allaah knows best.