Is it permissible for an old man to be alone with a female servant?
Question

                I am asking about the ruling on a female servant staying with my elderly grandfather after the death of my grandmother (his wife). My grandmother raised this servant from when she was young and she used to call them “mom” and “dad”, but not in a real sense. Now my grandfather is 75 years old and he lives alone after his wife died. Is it permissible for the servant to stay with him to serve him?.
Praise be to Allah.It is not 
    permissible for a man to be alone with a non-mahram woman, because of the 
    report narrated by al-Bukhaari (3006) and Muslim (1341) from Ibn ‘Abbaas 
    (may Allaah be pleased with him), that he heard the Prophet  (peace and 
    blessings of Allaah be upon him) say: “No man should be alone with a woman 
    without there being a mahram present, and no woman should travel unless she 
    has a mahram with her.” A man stood up and said: O Messenger of Allaah, my 
    wife has set out for Hajj and I have enlisted for such and such a campaign. 
    He said: “Go and do Hajj with your wife.” 
The fact 
    that the Prophet  (peace and blessings of Allaah be upon him) said “man” 
    is general in meaning and applies to every man, even if he is elderly. 
It says in
    al-Mawsoo’ah al-Fiqhiyyah (29/295): The majority of fuqaha’ say that 
    it is not permissible for a man to be alone with a non-mahram woman, because 
    the shaytaan will be the third one present, and he will whisper to them when 
    they are alone together to do that which is not permissible. The Prophet 
     (peace and blessings of Allaah be upon him) said: “No man is alone with 
    a woman but the shaytaan is the third one present.” The word “man” in this 
    hadeeth includes both old and young men, just as the word “woman” includes 
    both young and old women. End quote. 
If your 
    grandfather needs someone to serve him because he is old, he can choose 
    whichever of the following options suits him best, so that he will not be 
    alone with a woman in the way that is haraam: 

    1-He could marry this woman who 
    is serving him, or another woman, so that her staying in the house with him 
    will not involve anything that goes against sharee’ah. The scholars (may 
    Allaah have mercy on them) said that one of the purposes of marriage is to 
    serve the husband, as in the case of your grandfather. See al-Sharh 
    al-Mumti’ (10/383).

    2-One of his children or 
    grandchildren could serve him, so that he will have no need of this woman’s 
    services.

    3-If none of his children can 
    serve him, then at least one of them could be present in the house at the 
    times when the female servant is present, so that he will not be alone with 
    her.

    4-If that is not possible either, 
    he can agree with the servant that she will come at a certain time, and he 
    can go out of the house at that time, to buy whatever he needs, or to go and 
    pray in the mosque and stay there for a while, or visit a friend or 
    relative, and so on.
All of these 
    are means of avoiding that which Allaah has forbidden.
We ask 
    Allaah to help him to do that which is good and to protect him from sin.  
And Allaah knows 
    best.