What is required in ghusl in the case of janaabah is to wash the entire body with water, except that the woman whose hair is in braids is not obliged to undo them, but she should pour the water on her head in such a manner that it will reach all of her hair.
That is because of the reports narrated by Muslim (330) from Umm Salamah (may Allaah be pleased with her) who said: I said: O Messenger of Allaah, I am a woman with tightly braided hair; should I undo it for ghusl from janaabah? He said: “No; it is sufficient for you to pour three handfuls of water over your head, then pour water over you, and you will become pure.” According to another report: Should I undo them for (ghusl following) menses and janaabah? He said: “No.”.
Al-Nawawi (may Allaah have mercy on him) said in Sharh Muslim: Our view and that of the majority is that if a woman has her hair in braids and the water reaches all of her hair, inward and outward, without undoing the braids, she does not have to undo her braids, but if the water cannot reach all of her hair without undoing the braids, then she must undo them. The hadeeth of Umm Salamah is to be interpreted as meaning that the water reached all of her hair without her undoing it, because making the water reach all parts is obligatory. It was narrated from al-Nakha’i that it is obligatory to undo (braids) in all cases; it was narrated from al-Hasan and Tawoos that it is obligatory to undo braids when doing ghusl following menses, but not in the case of janaabah. Our evidence is the hadeeth of Umm Salamah. End quote.
If using water will harm her and lead to her hair falling out, then she should do tayammum and wash the rest of her body, because Allaah says (interpretation of the meaning): “But if you are ill or on a journey, or any of you comes after answering the call of nature, or you have been in contact with women (i.e. sexual intercourse), and you find no water, then perform Tayammum with clean earth” [al-Maa’idah 5:6]. This verse indicates that the sick person who will be harmed or his recovery delayed by using water should do tayammum. Allaah has explained the wisdom behind this legislation in the verse where He says (interpretation of the meaning): “Allaah does not want to place you in difficulty, but He wants to purify you, and to complete His Favour to you that you may be thankful” [al-Maa’idah 5:6].
If she can wipe her head, she should do that and do tayammum and wash the rest of her body, because she is able to do that and because Allaah has not burdened any soul beyond its scope.
Shaykh Ibn Baaz (may Allaah be pleased with him) was asked: I am a married woman and I am sick with an allergy in my chest, and I suffer fromt his condition throughout the year. How should I pray? Can I do ghusl without washing my head, and wiping it only? Please note that I get this problem when I wash my head several times a week, and I often do not pray because I am unable to wash my head and can only wipe it. I am confused and very anxious and worried, even though I know that the religion is easy. I hope that you can give me a definitive answer, so that I can live in peace and obey Allaah and fulfil my obligations properly. Please note that I am a teacher and I go out to work every day, and if I go outside after having a shower I fall sick, and Allaah knows that I am confused about how to live my married life, which means obeying my husband and above all obeying Allaah.
If you will be harmed by washing your head in the case of janaabah and following menses, it will suffice to wipe it and do tayammum too, because Allaah says: “So keep your duty to Allaah and fear Him as much as you can”[al-Taghaabun 64:16]. And the Prophet (blessings and peace of Allaah be upon him) said: “Whatever I forbid you to do, avoid it; and whatever I tell you to do, do as much of it as you can.” End quote from Fataawa Islamiyyah, 1/214.
For a description of tayammum, please see the answer to question number 21074.
This tayammum is permissible before or after ghusl, because washing the parts of the body in a particular order is not essential in ghusl.
It should be noted that some women exaggerate their fear about hair loss. It is essential that the hair loss should be a real sickness and not just something imagined or feared, in order for a woman to be allowed the concession of not washing her hair and wiping it and doing tayammum instead.
And Allaah knows best.