All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
Your husband has no doubt committed a grave sin if he actually committed adultery with that woman. You did well by advising him and urging him to repent of that sin. Extra-marital relationships incur such evil and corruption, and therefore Allaah The Exalted prohibited them as we have explained in Fatwa 81356.
Regarding men and women who have committed adultery, if they get married after sincere repentance and all the conditions of a valid marriage are fulfilled, namely the consent of the bride's Wali (legal guardian) and presence of the witnesses, then it is a valid marriage according to scholarly consensus. If they get married before making repentance, then there is a difference of opinion among the scholars regarding the validity of their marriage. Some jurists held that it is valid. For more benefit, please refer to Fatwa 83176.
When the husband divorces his wife for the first or second time, the divorce is revocable. During the ‘Iddah, the woman is still his wife. If he says or does something that indicates a revocation of the divorce before the ‘Iddah expires, then the divorce is revoked and their marriage bond is retained. Scholars held different opinions regarding the valid means of the revocation of a divorced wife; please refer to Fatwa 89845.
And for more benefit, please also refer to Fatwa 82541 about the types of divorce.
We would like to advise you not to preoccupy yourself with thinking about your husband's marriage with this woman. If he falls short in fulfilling your rights, you are entitled to demand your rights from him. If remaining married to him harms you, then you have the right to ask for divorce to ward off this harm. It is advisable, though, not to opt for divorce unless the benefits brought about as a result outweigh the harms incurred by the divorce. Please refer to Fatwa 225223.
Please note that it is impermissible for a Muslim woman to ask for divorce for no valid reason. The fact that her husband takes another wife does not give her the right to ask for divorce.
Finally, we would like to underline the following points:
- Polygyny is allowed in Islam on condition that the man is able to be just with his wives. Any man-made law that contradicts the Islamic law is simply worthless and should be disregarded. It is odd that the man is allowed to have illegitimate extra-marital affairs as he wishes and is prevented from taking a second wife. Please refer to Fatwa 82124.
- The husband should not threaten his wife with divorce if she stands in the way of his second marriage; this is improper for a Muslim man and runs counter to the noble Islamic manners.
- If the wife is negligent in her religion, the husband must give her advice. If she repents, then all is well; otherwise, it is better for him to divorce her.
Allaah Knows best.