Praise be to Allah.
If a man is able to marry a second wife, physically and financially, and he can treat both wives in a just manner, and he wants to take a second wife, then he is allowed to do so according to Islam. Allaah says (interpretation of the meaning):
“… then marry (other) women of your choice, two or three, or four…” [al-Nisaa’ 4:3]
And this was the practice of the Prophet (peace and blessings of Allaah be upon him), and of his Companions (may Allaah be pleased with them), but apart from the Prophet, no one is permitted to have more than four wives.
It is well known that women are by nature jealous and reluctant to share their husband with other women. Women are not to be condemned for this jealousy, for it existed in thebest of righteous women, the Sahaabiyyaat, and even in the Mothers of the Believers [the wives of the Prophet (peace and blessings of Allaah be upon him)]. But women should not let jealousy make them object to that which Allaah has prescribed, and they should not try to prevent it; a wife should allow her husband to marry another woman for this is a kind of cooperating in righteousness and piety. According to a hadeeth whose authenticity is agreed upon, the Prophet (peace and blessings of Allaah be upon him) said: “Whoever pays attention to his brother’s needs, Allaah will pay attention to his needs.”
The first wife’s consent is not a prerequisite for a man to take another wife. The Standing Committee for Issuing Fatwas was asked about this and replied as follows:
“It is not obligatory for the husband, if he wants to take a second wife, to have the consent of his first wife, but it is good manners and kindness to deal with her in such a manner that will reduce the hurt which women naturally feel in such situations. This is done by being kind to her and speaking to her in a gentle and pleasant manner, and by spending whatever money may be necessary in order to gain her acceptance of the situation.”
Concerning her request for divorce if her husband wants to marry another wife, this is a mistake. But they should examine the situation, and if she really cannot cope with living with another wife, then she can ask him for khula’ [ a kind of divorce instigated by the wife, whereby she forgoes the mahr]. If she can cope with living with the second wife, but it hurts her to do so, then she should be patient and seek the pleasure of Allaah. Thawbaan (may Allaah be pleased with him) narrated that the Messenger (peace and blessings of Allaah be upon him) said:
“No woman asks her husband for a divorce for no reason, but the fragrance of Paradise is forbidden for her.” (Narrated by Abu Dawood and others, and classed as saheeh by al-Albaani, may Allaah have mercy on him).
If she bears it with patience, then Allaah will make it easier for her and will expand her chest (i.e., grant her peace and calm), and will compensate her with something good. The husband must also help her by treating her kindly, being patient with her for any jealousy etc. on her part, and overlooking her mistakes. And Allaah is the source of help.