She asked her husband for a divorce because he mistreated her

Praise be to Allah.

Firstly: 

We ask Allaah to help you to do all that is good, and to bless you with happiness in this world and in the Hereafter, and to compensate you with a righteous husband and good offspring. 

You should note that tests and trials are part and parcel of the life of this world, and that the Muslim must prepare himself to face hardships and problems. One of the greatest trials that people may face is the loss of a loved one or friend through death, absence or separation, but Allaah, by His grace and kindness, still opens the gates of His mercy to people and makes it easy for them to find loved ones and friends who will help them to do good and reduce the sorrow that they feel in their hearts. 

Do not grieve for what has happened, for whatever happens to a person could not have missed him. Allaah, may He be exalted, says (interpretation of the meaning): 

“and it may be that you dislike a thing which is good for you and that you like a thing which is bad for you. Allaah knows but you do not know”

[al-Baqarah 2:216]

Secondly: 

It is permissible for a woman to ask her husband for a divorce, if there is a reason for that to be permissible, such as if he fails to give her her rights or he wrongs her or mistreats her, and does not respond to advice to treat her well and be kind to her. 

But if a woman asks for a divorce with no reason, that is haraam and is a major sin. 

The Prophet (peace and blessings of Allaah be upon him) said: “Any woman who asks her husband for a divorce for no reason, the fragrance of Paradise will be forbidden to her.”

Narrated by Abu Dawood (2226) and al-Tirmidhi (1187); classed as saheeh by al-Albaani in Saheeh Abi Dawood. 

Al-Mubaarakfoori said: i.e., without there being strong reason that compels her to seek a separation. 

Tuhfat al-Ahqadhi, 4/410 

Al-Haafiz Ibn Hajar said:  

The reports which warn a woman against asking her husband for a divorce are to be understood as referring to cases where there is no reason for doing that. End quote from Fath al-Baari, 9/402 

See also questions no. 9481, 12496 and 34579. 

Asking for khula’ is also permissible if it is done for a shar’i reason, so as to ward off wrongdoing or to protect one's rights or other such reasons. This has already been explained in the answer to question no. 1859. 

What is haraam is asking for khula’ with no good reason. 

It says in a hadeeth that the Prophet (peace and blessings of Allaah be upon him) said: “Women who seek khula’ are hypocrites.” Narrated by al-Tirmidhi (1186) and al-Nasaa’i (3461), both of whom classed it as da’eef. Ibn Qudaamah said in al-Mughni (7/248): Ahmad mentioned it and quoted it as evidence, and Ibn Hajar was of the view that it is saheeh, as it says in Fath al-Baari, 9/403. It was also classed as saheeh by al-Albaani in Saheeh Sunan al-Tirmidhi, 1186. 

Al-Mubaarakfoori said: 

i.e., those women who ask for khula’ and divorce from their husbands for no reason. End quote. 

Tuhfat al-Ahwadhi, 4/409 

Thirdly: 

What the husband must do, if he marries more than one wife, is to adhere to the condition that Allaah has stipulated for husbands, as He says (interpretation of the meaning): 

“but if you fear that you shall not be able to deal justly (with them), then only one”

[al-Nisa’ 4:3]

If he knows that he will fall short in giving one of his wives her rights, then it is not permissible for him to keep her suspended and mistreat her, rather he must give her her rights of maintenance, staying overnight with her and treating her in a kind and proper manner, and he must be just and fair between her and his other wives. If he cannot do that, then he should let her go with kindness, and kindness means that he should not take anything of her wealth. 

See also question no. 45600. 

If the husband refuses to let her go with kindness and he does not care about that wrong that is being done to his wife, and the wife wants to expedite her separation from him, even by paying some kind of compensation to the husband, then she has the right to do that, and the sin is on the husband and the money that he takes is haraam wealth. 

We have already discussed that in detail in question no. 42532. 

Fourthly: 

With regard to the question as to whether divorce shakes the Throne of the Most Merciful: 

There is no saheeh report concerning that. The hadeeth which is narrated about that is fabricated and false. 

We have explained that in the answer to question no. 43498  

And Allaah knows best.