She wants to have surgery in order to lose excess weight

Praise be to Allah.

We have previously explained on our site that some kinds of cosmetic surgery are permissible, namely that which is done to treat a fault, remove a deformity and so on, which comes under the heading of medical treatment. And some kinds are haraam and are not allowed, namely that which is done for the purpose of beautification, because it is changing the creation of Allah. 

It was narrated from Ibn Mas‘ood (may Allah be pleased with him) that the Messenger of Allah (blessings and peace of Allah be upon him) said: May Allaah curse the one who does tattoos and the one who has a tattoo done, the one who plucks eyebrows and the one who has her eyebrows plucked, and those who file teeth for the purpose of beautification, changing the creation of Allaah. Narrated by al-Bukhaari (5931) and Muslim (2125). 

Al-Haafiz Ibn Hajar (may Allah have mercy on him) said:

From this it may be understood that the one who is blameworthy is the one who does that for the sake of beauty; if she needs to do that for the purpose of medical treatment, for example, it is permissible. End quote. 

The scholars of the Standing Committee for Issuing Fatwas were asked: 

What is the ruling on cosmetic surgery, which is doing surgery by means of which the patient may get rid of a specific deformity, and the doctor may change some of the physical features of the patient. Is this changing the creation of Allah? 

They replied: It is permissible to do the surgery mentioned and it is not regarded as changing the creation of Allah. End quote. 

Fataawa al-Lajnah al-Daa’imah (25/55). 

According to medical reports, excess weight and obesity affect one’s health and put one at risk of disease. We ask Allah to grant well-being to us and you and all the Muslims. 

If it is not possible to treat this excess weight by regular means, such as reducing food intake or following a healthy diet program or beneficial medicines or exercise and the like, then surgical intervention may be the last resort in treating it. If a trustworthy Muslim doctor who is skilled in weight reduction surgery has advised it, then there is nothing wrong with your doing that surgery as has become essential due to your circumstances and need. 

For more information please see the answer to question no. 97651 and 102085. 

And Allah knows best.