How can a woman purify herself from ritual impurity (janabah) if her doctor has advised her not to wash her hair more than once a week due to hair loss?

The obligation in ghusl al-janabah (ritual bath after sexual defilement) is to ensure water covers the entire body. A woman is not required to undo her braids if the water reaches all of her hair, based on the hadith of Umm Salamah (may Allah be pleased with her). If the use of water harms her and leads to hair loss, she should perform tayammum (dry ablution) and wash the rest of her body. If she is able to wipe her head, she should do so, then perform tayammum and wash her body. Indeed, the sick person whom water harms should perform tayammum, for Allah does not burden a soul beyond its capacity. Sheikh Ibn Baz issued a fatwa permitting wiping the head and performing tayammum if washing it would cause harm. The hair loss must be a genuine medical condition to permit this for the woman.