Praise be to Allaah.
Allaah has caused men to be attracted to women, and women to be attracted to men, and this inclination sometimes results in haraam relationships such as zina, and sometimes it results in permissible relationships such as marriage. Allaah has made the wife a covering for her husband and has made the husband a covering for his wife. Allaah says (interpretation of the meaning):
“They are Libaas [i.e. body-cover, or screen, or Sakan (i.e. you enjoy the pleasure of living with them)] for you and you are the same for them”
One of the blessings that Allaah bestows upon some spouses is that He creates love and compassion between them, and helps each of them to do that which causes harmony to grow in the family and ward off disputes and arguments. This is a very great blessing indeed, which is not appreciated by anyone but those whose family relationships are disrupted and disputes and arguments arise amongst them, which turn the marital relationship into an unbearable hell. If that happens, each spouse starts to dream of a stable family life, and the man wishes for a wife with whom he can live in peace, and the wife wishes for a man with whom she can live in peace.
From your question, it may be understood that Allaah has bestowed all these blessings upon you, so what you should do is be grateful to Allaah for this great blessing and strive to preserve it and the family with which Allaah has blessed you, for millions of women wish that they could be in the good situation in which you are, but you do not appreciate its value.
You should note that it is not permissible for a woman to form a relationship with a non-mahram man. If she is married, then such a relationship is even more haraam, because it is a transgression against the husband’s rights and honour.
Based on this, it is not permissible for you and this evil lover to pray istikhaarah, because istikhaarah is only prescribed in cases where it is not clear if the matter is good or bad, and the Muslim does not know where his interests lie, so he prays istikhaarah asking that Allaah will help him to attain that which is good if it is good, or divert it from him if it is bad. But if a Muslim prays istikhaarah with regard to disobeying Allaah or going against His commands, this is a sin for which he has to repent to Allaah.
To explain further: if a Muslim woman prays istikhaarah about marrying someone other than her husband when she is still married to her husband, then in fact she is praying istikhaarah about wrecking her home and family, and praying istikhaarah about hurting her children, and praying istikhaarah about divorcing a husband who is treating her well and taking good care of her. So she is praying istikhaarah about betraying him and stabbing him in the back by tearing apart his family, so that his home and hers will be destroyed at her hands. She is praying istikhaarah about responding to great kindness and good treatment with a great wrong and denying the rights of one who has treated her well.
All of these factors and many others apply to the istikhaarah prayer that you offered.
As for the positive result that you say your friend got, undoubtedly this is a case of the shaytaan making following one's whims and desires appear attractive. The Muslim should not pray istikhaarah about doing something that is haraam, so how can he pray istikhaarah and claim that he got a positive result?!
Moreover, after praying istikhaarah the Muslim should resolve firmly either to do it or not to do it. Whatever Allaah makes easy for him is what is good, but if he waits until he feels good about it or sees a dream and the like, these are usually illusionary matters on which no shar’i ruling can be based.
Based on the above, you should push away all whispers from the shaytaan that have to do with this matter, and do not give evil any way of reaching you or your family or children. You should realize that you have fallen into a trap of the shaytaan, because he has made you attractive to this young man and has made him attractive to you so that he may achieve what is his greatest dream, namely the destruction of a stable, believing Muslim family and the divorce of two spouses who love one another, and the neglect of their children.
Thwart the plans of the shaytaan by not letting this young man destroy your life and your family. Cut off all means that may enable him to continue to have any place in your life.
One of the ways in which you can ward off these devilish whispers is to ask yourself the following questions and answer them honestly:
1. If this young man is righteous, then how can he accept to destroy the home of his Muslim brother and split up his family?
2. If this person really loves you, why is he striving to wreck your home and destroy your family? Does he love you or does he love himself and is only looking out for his own interests and desires?
3. If this young man were to get what he wants and you were to get divorced from your husband – Allaah forbid – what would be the fate of your children who are a trust about which Allaah will question you on the Day of Resurrection?
4. What guarantee do you have that this young man will treat you as nicely after marriage as he does now? It should be noted that many marriages that are based on “love” are doomed to failure after only a few months, because they are based on a weak foundation, not on a foundation of pleasing Allaah and His Messenger (peace and blessings of Allaah be upon him).
5. Do you expect that trust will last between you after you get married? If he loves you although you are married, then how can you be sure that he will not fall in love with someone else who is also married or not married? How can he trust you if you destroyed your home for his sake? This pattern may be repeated when you are married to him. These doubts will persist and will be a cause of anxiety for both of you. You have both accepted something haraam and you have not refrained from establishing a haraam relationship even though there is a legitimate marriage contract between you and your husband, so who can guarantee that it will not happen again?
With regard to your request for a du’aa’, I ask Allaah, the Most High, the Almighty, by His grace and favour, to make goodness easy for you and to ward off all evil from you, and to continue to bless you and your family with stability and love, and to keep you and your husband and children safe, and to ward off from you the whispers of the shaytaan and his making falsehood attractive.And Allaah knows best.