All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
Whoever assumes the state of Ihraam for Hajj or ‘Umrah is prohibited from cutting his hair or taking anything from it before finishing the rites of Hajj or ‘Umrah, whether man or woman. Ibn Al-Munthir said in his book Al-Ijmaa’: “The scholars have agreed with consensus that a person in a state of Ihraam is prohibited from the following: having sexual intercourse, killing wild game, wearing perfume, wearing certain kinds of clothes, and cutting the hair and nails.”
As-Sarakhsi said in Al-Mabsoot: “Removing what grows from the body is forbidden for a person in the state of Ihraam.”
Therefore, if a person takes anything from his hair, he is obliged to make the expiation. The required expiation for removing the hair of the head is either fasting three days or feeding six poor persons – half a Saa' for each poor person - or slaughtering a sheep.
Ibn Qudaamah (from the Hanbali school of jurisprudence) said: “The expiation is one of the three that are mentioned in the verse and in the Hadeeth; he may do whichever of them he wishes, because he is commanded to do one of them according to his choice, and it makes no difference whether or not one has an excuse [to violate the Ihraam] or whether one does it deliberately or by mistake. This is the view of Maalik and Ash-Shaafi’i. Ahmad said (in another narration from him) that if one shaves without a valid excuse, then he is obliged to offer a sacrifice and does not have the choice [to do one of other three matters]. This is also the view of Abu Haneefah.”
However, the jurists differed in opinion about the amount (of hair) that if cut renders the expiation obligatory. Some of them say three hairs or more, as per the view of the Hanbalis and Shaafi’is, and some of them say one-quarter of the head, as per the opinion of the Hanafis.
As for a man cutting his wife's hair for her to come out of Ihraam, then this is acceptable whether he does so before or after he himself comes out of Ihraam. The Hanafis, however, hold that it is forbidden for one who is in the state of Ihraam to cut the hair of another before he himself comes out of Ihraam; and if he does so, he is obliged to pay charity according to their view.
Badaai’ As-Sanaa’i (a Hanafi book) reads: “If a person (in the state of Ihraam) takes anything from his head or beard, or touches one of them and a hair falls out because of that, then he is obliged to pay charity for relieving himself by removing hair. This is if he shaves his own hair. If he shaves another person's hair, then according to our School, the one who shaves the hair of another person is obliged to pay charity, whereas Maalik and Ash-Shaafi’i say: “The person who shaves the hair of another is not obliged to do anything.” Their view is that the legal basis of the expiation is relieving one's self by removing hair, which does not occur when one shaves another. In our view, since a Muhrim is prohibited from shaving his own hair, then he is also prohibited from shaving the hair of others as Allaah says (what means): {And do not shave your heads until the sacrificial animal has reached its place of slaughter.} [Quran 2:196] One usually would not shave his own head, but since it is forbidden for him to shave the heads of others, then it is for forbidden for him to shave his own head with greater reason. Therefore, he is obliged to give charity but not to offer a sacrificial animal as he does not relieve his own self by doing so. This goes for whether or not the shaved person is in the state of Ihraam or not as discussed earlier...”
Finally, it should be noted that it is forbidden for a Muhrim to touch his wife with lust before coming out of Ihraam, so if he cuts his wife's hair before he comes out of Ihraam, then he is not permitted to touch her hair lustfully.
Allaah Knows best.