Firstly: 

In the answer to question no. 1171, we stated that it is haraam to use wigs in principle, even if it is for a woman to adorn herself for her husband, and that it comes under the heading of hair extensions, which are haraam. 

In the answers to questions no. 101430 and 113548, we stated that it is permissible to wear a wig in order to cover a defect in the case of a woman who is bald, and that this is the view of Shaykh al-‘Uthaymeen (may Allah have mercy on him), unlike other contemporary scholars who regard that as coming under the same heading as hair extensions which are haraam. The words of Shaykh al-‘Uthaymeen may be found in the questions referred to. 

Secondly: 

With regard to wiping over the wig when doing wudoo’, it is better and more on the safe side to remove it when doing wudoo’ and to wipe the head directly. This is more on the safe side, even though wiping over it is permissible. The Prophet (sa) wiped over his hair when it was held together with a sticky substance (mulabbad), and he wiped over his turban. 

But in the case of ghusl, it must be removed so that the water can reach all of the skin. 

Shaykh Muhammad ibn Saalih al-‘Uthaymeen (may Allah have mercy on him) said: 

The scholars differed with regard to whether it is permissible for a woman to wipe over her head cover. 

Some of them said that it is not acceptable because Allah enjoined wiping the head when He said (interpretation of the meaning): “rub (by passing wet hands over) your heads” [al-Maa’idah 5:6]; and if she wipes over her head cover, she has not wiped over her head, rather she has wiped over a barrier, namely the head cover, so it is not permissible. 

Others said that it is permissible, and they drew an analogy between the head cover and the man’s turban: the head cover for women is like the turban for men, and difficulty is present in both cases. 

Whatever the case, if there is any difficulty because of cold weather, or in taking it off and wrapping it again, then there is nothing wrong with wiping over it in that case, although it is better not to wipe over it, and there is no saheeh hadeeth in this regard. 

If the hair is stuck together with henna or dye or honey and the like, it is permissible to wipe over it, because it is proven that the Prophet (blessings and peace of Allah be upon him) used to stick his hair together with something when he entered ihram, and whatever is applied to the hair in this fashion is regarded as being part of the head. 

This indicates that there is some element of leniency with regard to purifying the head. 

Based on this, if a woman has stuck her hair together with henna, it is permissible for her to wipe over it, and there is no need for her to undo her hair and scrape away this henna. 

It may be said that there is a precedent, namely the ring; the Messenger (blessings and peace of Allah be upon him) used to wear a ring, but despite that he did not make the water go between the ring and the skin. Such matters may be overlooked in sharee‘ah, especially since the basic principle concerning the head is that it does not have to be purified by washing; rather it is purified by wiping. Hence its purification is reduced to wiping. 

The turban, khuff (leather slipper) and head cover may only be wiped in the case of minor impurity, not major impurity. The evidence for that is the hadeeth of Safwaan ibn ‘Assaal who said: The Messenger of Allah (blessings and peace of Allah be upon him) instructed us when we were travelling not to take off our khufoof (leather slippers) for three days and nights, except in the case of janaabah, but not in the case of defecation, urination or sleep. 

The words “except in the case of janaabah” refer to major impurity. 

The words “but not in the case of defecation, urination or sleep” refer to minor impurity. If a person becomes junub during the period when wiping over the khufoof is permitted, then he should not wipe over them; rather he has to do ghusl, because in the case of major impurity, there is nothing that can be wiped over at all, except a plaster cast. End quote. 

Al-Sharh al-Mumti‘ ‘ala Zaad al-Mustaqni‘, 1/239-242. 

And Allah knows best.