All perfect praise be to Allaah, the Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
The Messenger of Allaah, sallallaahu ‘alayhi wa sallam, said, “A woman in the state of Ihraam does not wear Niqaab or gloves.” [Al-Bukhari] But it is permissible for her to let her head-covering garment drape from her head down over her face when non-Mahram (i.e., marriageable) men pass by her.
If she covers her face with a Niqaab for an excuse such as weak-sightedness, then she is liable to a Fidyah (expiation or ransom) because of committing a violation of Ihraam. Allaah The Almighty Says (what means): {And whoever among you is ill or has an ailment of the head [making shaving necessary must offer] a ransom of fasting [three days] or charity or sacrifice.} [Quran 2:196]
Thus, she has the option to do either of three things: fast for three days, or feed six needy people half a Saa’ (double handful) for each, or slaughter a sheep and distribute the meat to the poor people living in the Sacred Sanctuary.
Allaah Knows best.