This issue is clarified in the following fatwa issued by Sheikh Faysal Mawlawi. He said:

“There is a unanimous agreement among Muslim scholars that washing the hair is obligatory for both men and women when performing Ghusl. The majority of Muslim scholars state that the woman who has braids is NOT obliged to untie them when performing Ghusl. Rather, she can pour water over them till water reaches the roots of the hair and the scalp. The Prophet, peace and blessings be upon him, is reported to have addressed Umm Salamah, may Allah be pleased with her saying, “It is sufficient to pour water over your head three times and then pour it over your whole body three times, and with that your Ghusl is completed.” (At-Tirmidhi)

Thus, what is to be done is washing the hair till the water reaches the scalp, and it is not sufficient to wipe over the hair without washing it thoroughly unless there is a clear sign that washing the hair will cause physical harm for the woman. A reliable physician determines such case, and this gives a woman an excuse to wipe over the hair, without washing.”

It has become clear that washing the head thoroughly is obligatory in Ghusl; the woman has to wash her hair not to wipe over it without necessity as stated above.