He is in love with a girl and she loves him but he says that they are doing something good
Question

                Allaah has guided me but in a strange way. What encouraged me to do that was a woman. She nearly destroyed me; she loved me but it was not a permissible kind of love. I got to know her through the internet, and I ask Allaah to forgive me for that. Now she is advising me and I am afraid that if I leave her she will do something to herself. I am also afraid that if she gets married and there are problems between her and her husband, she will say, “So and so is better than you, I wish that I had married him,” meaning me. How often I tried to leave her but I could not, for one reason, which is that we are attached to one another, the reason being that we get along and we are cooperating in righteousness and piety; she obeys me in anything that is pleasing to Allaah. I want to marry her, but she is engaged to one of her relatives and her family refuse to entertain the idea of her marrying a stranger. Every time I get away from her I feel that I am alone, and the Shaytaan comes to me when I am alone and tempts me. When I speak to her I feel at ease, because she reminds me of Allaah. Please help me, may Allaah have mercy on you.
Praise be to Allah.
    You should not hesitate to give up this girl and keep away 
    from her. The Shaytaan has clearly confused you both; he has made you do 
    something haraam, and has made it appear attractive to you by making you 
    think that it is obedience and drawing closer to Allaah. 
What you say in your 
    question is indicative of what we are saying. You say, “[Allaah] has guided 
    me”, “she is advising me”, “she obeys me in anything that is pleasing to 
    Allaah”, “we are cooperating in righteousness and piety” and “she reminds me 
    of Allah”. All of these are words that are acceptable in Islam, but the 
    Shaytaan has made them into lovers’ words.  

    We are certain that you would not agree for any of your own 
    sisters or daughters to enter into such a relationship, would you? So why do 
    you think that it is acceptable for other people’s daughters? 

    Do you think that if your own daughter did something like 
    this, corresponding and speaking to a non-mahram man when she is engaged to 
    one of your relatives, that you would approve of her actions, even if she 
    were to say to you, “Allaah has guided this young man at my hands, and I 
    remind him about Allaah, and we are cooperating in righteousness and piety, 
    and I am advising him”?! 

    I do not think that you would like it if your fiancée were to 
    correspond with a young man, or if a young man were to correspond with her 
    on the grounds of “sincere advice” or “cooperating in righteousness and 
    piety” etc. 

    No, by Allaah, we do not think that any wise person would 
    accept this, so do not be deceived by the Shaytaan’s making your deeds 
    appear attractive to you. End this relationship immediately and let this 
    girl go; do not worry about what she may say to her husband, because you are 
    not responsible for her and you have nothing to do with her. You don’t know 
    – perhaps she will marry someone who is better for her than you! 

    You say that she is engaged, and the Prophet  (peace and 
    blessings of Allaah be upon him) said: “No man should propose to a woman to 
    whom his brother has already proposed marriage.” (Agreed upon). So it is not 
    permissible to do things the right way and approach her family, asking to 
    marry her; so how can it be permissible for you to speak to her and 
    correspond with her without her family knowing? 

    Fear Allaah, and leave her without any hesitation. Do not get 
    in touch with her again. Leave her and her business and her family alone. 
    Any woman who agrees to speak to non-mahram men when she is engaged or 
    married cannot be entrusted with a household or with the upbringing of her 
    daughters and sons. Beware lest Allaah punish you with regard to your own 
    family and children. I ask Allaah to guide you and help you. Give this up 
    for the sake of Allaah and Allaah will compensate you with something better 
    than that which you have given up. 

    See also the answer to questions no. 
    47405 and 
    36618 

    We ask Allaah to help you to do that which is best for you in 
    this world and in the Hereafter. 

    And Allaah is the Source of strength.