Betrothing a married woman is an unlawful act
21-2-2012 | IslamWeb
Question:
There is a married woman whose marriage lasted only a few months and then disagreement occurred between her and her husband causing her to file for a divorce. At that time, she was acquainted with a young man by chance, who was a friend of her cousin's husband and he offered her marriage claiming that he loved her. Consequently, they talked to each other over the telephone although she was still married. My question is: What is the Sharee‘ah ruling regarding this young man whom I describe as dissolute, as he exploited the vulnerable emotional state of that woman?
Answer:
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 
is His slave and Messenger.
It is narrated that Ibn ‘Umar 
said: "None of you should betroth a woman whose brother (in Islam) has betrothed." [Al-Bukhari and Maalik: Marfoo']
This contradicts superior morals and chivalry, afflicts the first suitor with harm and brings about hatred among people. Some scholars said that a marriage should be nullified if it is concluded on such a betrothal.
Consequently, if Islam prohibits betrothing a woman already betrothed by another man, then what will be the ruling in case of betrothing a woman who is married to another man? This is certainly prohibited. Even if she has filed for a divorce before a judge, no man is permitted to betroth her. Moreover, even if the judge has finalized the divorced and such a divorce has been documented, it is still not permissible for anyone to betroth her before the end of her ‘Iddah (post-marriage waiting period).
Islam also prohibits spoiling relationships between people, especially spouses. Buraydah 
narrated that the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, said: "Whoever swears by Amaanah (honesty) is not of us, neither is he who instigates a wife against her husband or a slave against his master". [Ahmad]
Thus, the behavior of this young man is unlawful under the Sharee‘ah (Islamic law). He must fear Allaah The Almighty and withdraw his proposal to that woman.
Allaah Knows best.