She used to sit with him and his wife, and he decided to marry her and divorce his wife!
Question

                My problem is that I am about to marry the husband of my sister’s friend. I used to go to her house and sit with her and her husband, and we would always talk with one another. When my sister tried to advise her that this is haraam, i.e., mixing, she made fun of my sister and told her “you are backward” until her husband came and proposed to me, and he says that he liked me from the first moment he saw me, and he wants to have children because she cannot have any. When she heard about this, she said that I had betrayed her. Should I agree to be his wife or not? He wants to divorce her because they have arguments. Please note that he works in the bank and wants to change his job because he knows that it is haraam; he always prays in the mosque.
Praise be to Allah.Firstly: 
Mixing between men and 
    women is haraam. See the answer to question no. 
    1200. 
What you must do is repent 
    to Allaah from what has happened of that, and resolve not to do it again in 
    the future. 
Secondly: 
It is haraam for a woman to 
    strive for her Muslim sister to be divorced, because the Prophet  (peace 
    and blessings of Allaah be upon him) said: “no woman should ask for her 
    sister to be divorced so as to deprive her of what is rightfully hers and 
    take it for herself.” Narrated by al-Bukhaari (2140) and Muslim (1413). So 
    it is not permissible to encourage or tempt the husband to divorce his wife. 
     
Thirdly: 
If the wish to marry came 
    from the husband, and you did not try to separate him from his wife, it is 
    permissible for you to accept, but we do not advise you to do that for 
    several reasons: 

    (i)There is no guarantee that this 
    man will not like someone else and choose her over you as he did with his 
    first wife.

    (ii)Your agreeing to marry him will 
    stir up resentment, hatred and enmity in the heart of his wife, and you will 
    not be safe from her ill-will and harm.

    (iii)What you have mentioned about 
    him working in the bank; he may leave it as he said he would, or he may not.
This is what we think. If, 
    after thinking long and hard about the matter, you decide to agree to marry 
    him, then pray istikhaarah and ask Allaah for guidance, and wait until he 
    finds another job that is permissible. 
We ask Allaah to help you 
    to do that in which there is goodness, success and guidance. 
And Allaah knows best.