Woman’s
    former boyfriend took her away from Islam
Question

                Assalamu Alaikum. In the event that a
      non-muslim woman who has been in a relation-ship with another non-muslim and then decides
      to leave him for a muslim because she realised that her boyfriend does not treat her well
      any-more and confesses to the muslim that she loves him and wants to be married to him.
      Based on her assurance that she does not love and does not wish to remain with her
      non-muslim boyfriend anymore, the muslim decides to accept her even-though she has
      committed fornication with her former boyfriend, because she decided to embrace Islam.
      Allah forgives all past sins of a new-believer.
  Then, she goes back on her words as her former boyfriend convinces her not
      to leave him when he realises that she's leaving him for another man(his friend-it's me).
      When this happend I was simply devastated. Here I was willing to accept her for what she
      is since she was willing to embrace Islam and she goes back on her assurances. How am I to
      Perceive this please? Furthermore, upon knowing that her boyfriend has been fooling around
      with other girls(not sure if he has been sleeping around), I tried to carelessly warn her
      without any prove that she was going to regret her decision about going back to him. Thus,
      I was pictured in a bad light for that. Was what I did wrong, trying to get her back,
      since I thought eventhough she may have a Blissful life with her former boyfriend in this
      life, Eternal punishment is awaiting for anyone who does not embrace Islam and die a
      believer? Please enlighten me. 
  May Allah's Mercy and Blessings befall you in this world and the next.
      Wassalam.
Praise be to Allah.It seems that you are confusing matters of the heart with matters of
    aqeedah and religion, because you state in your question that you were emotionally
    devastated when this woman left you and went back to her kaafir boyfriend. You need to
    keep matters of belief and religion separate from your emotions and personal desires.
    Adhere to the limits set by Allaah, and follow His rules, one of which is the aayah
    (interpretation of the meaning): Let no man guilty of fornication or adultery
    marry any but a woman similarly guilty, or an Unbeliever; nor let any but such a man or an
    Unbeliever marry such a woman; to the Believers such a thing is forbidden. [al-Noor
    24:3] It is not permitted to marry such a woman unless she repents, and this woman has
    not repented, she has gone back to committing haraam deed with a kaafir. So do not waste
    your time feeling regret about her. Ask Allaah to bless you with a chaste, believing wife,
    one who fasts and prays and devoutly adheres to the limits set by Allaah. This is the kind
    of woman you should long for, not this impure woman whose interest in Islam was only a
    temporary, emotional affair. I also advise you to adhere to the shareeah when
    calling non-Muslims to Islam, and not to treat the matter of speaking to non-mahram women
    (women to whom you are not related) lightly. You should not start any kind of forbidden
    contact or relationship with them in the name of dawah. The Muslim man should use
    indirect means of communication when calling non-Muslim women to Islam, such as giving
    them books or tapes, or communicating with them via e-mail, etc., within the bounds of
    what is right and proper. We ask Allaah to guide us all and make us content with what is
    halaal so that we will have no desire for anything that is haraam. May Allaah bless our
    Prophet Muhammad.