The 'awrah of women in front of other women
Women are obliged to cover themselves in a modest way in front of other women. This includes both Muslim and non-Muslim women. So she must cover herself properly.
Now, she may uncover her hair in front of Muslim women, but this cannot be the case with non-Muslim women.
In front of Muslim women, she can uncover: hair, forearms, neck, center of the clavicle (part of the upper chest), shins, and feet.
In front of non-Muslim women, she cannot uncover the same like with Muslim women. So she can only show her face and hands.
And tell the believing women to lower their gaze and to guard their chastity and not to show their beauty but what should be visible of it. And they should wear the veils completely over their bosoms and not display their beauty openly except to ... their fellow women ...
[Surah Nur verse 31]
Tafsir:
{their fellow women}
Imam Ahmad said:
"The women of the book (Jews and Christians), are not allowed to see the hair of the Muslim woman (so it should be covered for them). Makhoul (Tabi') and others said this."
[Ahkam an-Nisa page 38 athar 35, from Al Khallal]
Imam Ahmad said:
"The women of the book (Jews and Christians), are not allowed to see the head of the Muslim woman (so she must cover her face from them)."
[Ahkam an-Nisa page 38 athar 35, from Al Khallal]
This verse of Surah Nur refers to the Muslim women. Their fellow women can see her face, hand and hair. That would not be a problem. But those beauties cannot be uncovered in front of non-Muslim women.