A woman while making ablution is not required to untie her hair bun before wiping. It is sufficient to wipe with a wet hand over the front part of her hair. She is not supposed to make sure the water reach or touch the skin of her head when wiping. However, it is not sufficient just to touch the head with a wet finger.
Elaborating on how to wipe over head while performing ablution, Sheikh Sayyed Sabiq, states in his well-known book, Fiqh As-Sunnah, in this regard:
“Wiping the head means to wipe one’s head with his/her hand. It is not sufficient just to place the hand on the head or to touch the head with a wet finger. The apparent meaning of the Qur’anic words, “…and wipe over your heads…” does not imply that all of the head needs to be wiped. It has been recorded that the Prophet used to wipe his head three different ways:
1- Wiping all of his head. ‘Abdullah ibn Zaid reported that the Prophet (peace and blessings be upon him) wiped his entire head with his hands. He started with the front of his head, then moved to the back, and then returned his hands to the front. (Related by “the group.”).
2- Wiping over the turban only. Said ‘Amru ibn Umayyah, “I saw the Messenger of Allah, upon whom be peace, wipe over his turban and shoes.” (Related by Ahmad, al-Bukhari and Ibn Majah). Bilal reported that the Prophet (peace and blessings be upon him) said, “Wipe over your shoes and head covering.” (Related by Ahmad.) ‘Umar once said, “May Allah not purify the one who does not consider wiping over the turban to be purifying.” Many hadiths have been related on this topic by al-Bukhari, Muslim and others. Most of the scholars agree with them.
3- Wiping over the front portion of the scalp and the turban.
Al-Mughirah ibn Shu’bah said that the Messenger of Allah (peace and blessings be upon him) made ablution and wiped over the front portion of his scalp, his turban and his socks. (Related by Muslim.) There is, however, no strong hadith that he wiped over part of his head, even though verse 6 of surat al-Ma’idah apparently implies it. It is also not sufficient just to wipe over locks of hair that proceed from the head or along the sides of the head.”