Praise be to Allaah.
Firstly: the woman in ihram is not allowed to wear the niqaab
because the Prophet (blessings and peace of Allah be upon him) said: “The
woman in ihram should not wear niqaab or gloves.”
This has been discussed previously in the answer to question
Secondly: the prohibition on a woman wearing niqaab whilst in
ihram does not mean that she should leave her face uncovered in the presence
of non-mahram men; rather she must cover her face with something other than
the niqaab or burqa‘.
The Prophet (blessings and peace of Allah be upon him) did
not instruct the woman in ihram to uncover her face; rather he told her not
to wear the niqaab. There is a difference between the two matters; she
should not wear the niqaab but she should cover her face with something
other than the niqaab, such as the scarf.
It was narrated that ‘Aa’ishah (may Allah be pleased with
her) said: The riders used to pass us when we were with the Messenger of
Allah (blessings and peace of Allah be upon him) in ihram. When they drew
close to us, one of us would let down her jilbaab from her head over her
face, and when they had passed us we would uncover it again.
Narrated by Ahmad, 23501; Abu Dawood, 1833; classed as hasan
Shaykh al-Islam (Ibn Taymiyah) said: She may cover her face
and her hands, but without wearing the garment that is made to fit that
part, just as a man is not allowed to wear trousers but he may wear the
izaar (waist wrapper).
End quote. Majmoo‘ al-Fataawa, 22/120
This issue has been discussed previously in the answer to
question no. 120377
Thirdly: even though it was a mistake to uncover the face
after entering ihram, the ‘umrah is not affected by a woman uncovering her
face after entering ihram or whilst circumambulating the Ka‘bah (tawaaf) or
going between as-Safa and al-Marwah (sa‘i). The ‘umrah is valid and we hope
that it will be accepted in sha Allah, especially since your wife uncovered
her face due to lack of knowledge of the Islamic ruling.
And Allah knows best.