Assalamu alaikum wa rahmatulahi wa barakatuh sister,
Unfortunately, in today’s world there is a lot of pressure for girls to look a certain way and it seems your daughter has fallen into this trap. This is causing you great distress as you fear for her health. Alhamdulilah, Islam has prescribed a way of life that can detract us from falling into this kind of mentality, so the solution to this difficulty can be overcome by drawing on Islamic guidance.
From an Islamic perspective there are 2 things in particular that we can draw on. Firstly, taking care of ourselves and our bodys, which includes eating well. Allah has entrusted us with our bodys as a house for our soul. They do not belong to us, but are merely a temporary vessel for carrying our souls. Therefore, we have the responsibility to take care of them in this world and we will be questioned about the way we performed this role. One of the requirements for this is to eat well. The fear of Allah in this case should push her to take better care of her eating habits. Secondly, if her desire to look thin is for the purpose of pleasing others or feeling good about herself then she needs to be made aware of the importance of inner beauty over outer beauty.
That pleasing Allah by taking care of herself and being a good, righteous person are more valued qualities than being thin. Will Allah love her more if she is thing? Will her weight be what earns her a place in Jannah? You might also challenge her about how she would rather other people value her too; as a beautiful thin person or a good, righteous person? What qualities are more valuable to them and what she rather they value her on? Would she even want to be friends with someone who only judges her based on her weight? Would her friends think any less of her if she was not thin? You can also strengthen this point by putting the shoe on the other foot so to speak. Does she judge others by their appearance? Would she judge someone badly for being overweight? Would she choose to not be friends with someone because they were overweight? Or even just not thin? We she rather be friends with someone who was not thin, but who was a good person that she could trust and rely on? These are questions you could ask her to think about. You don’t necessarily have to actually discuss the answers with her as it may lead her to get angry and turn the conversation into an argument which will not make her any more likely to change her eating habits.
If she practises Islam well and her faith is strong then this will be relatively easy. However, if she doesn’t practise Islam much, or her faith is weak then it will be more challenging. In this case, you would need to encourage her to do so more, by being a positive role model and letting her observe you engage in the various daily acts of worship. This might encourage her to join you in prayer for example. As she gets more involved in her Deen, her heightened awareness of Allah will help her to see the light about what is most important. The fact is Allah asks us to cover anyway and generally it is not possible to even tell if someone is thin or overweight if clothes are worn in a modest way, not showing the shape of the body, so no one could eve tell if she were thin or not. So education on modesty in Islam once her levels of eman are at a stage that she will be receptive to this message will also help with her own perceptions about the importance of being thin or not.
Let her know that you care about her and that is why you are concerned about her eating habits. This way she will feel less like you are nagging her and more like you are simply concerned for her health. She will be more likely to listen and make changes if she feels it is due to your concerns as she will feel cared about. Let her know that she is beautiful as she is and highlight her good qualities. Make her feel confident about aspects of herself that are not related to her weight. This sense of confidence in herself will make her comfortable to be who she is without the need to be thin or worry about her weight. Encourage her to engage in activities that she is good in to ensure that her levels of self-esteem are high and she will be less fixated on being thin.
So, there a number of different ways you can approach this scenario. May Allah guide her on the straight path and help you to support her to make the best decisions.