Wa `alaykum As-Salamu waRahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
Answering your question, Dr. Mohammad S. Alrahawan, Associate Professor at the department of Islamic Studies in English, Al-Azhar University, Egypt, states:
The area to be covered by a woman when being attended by another woman is what is between the navel and the knee, whether the woman is her mother or sister or is not her mahram. This is the view of the majority of Muslim scholars.
It is not permissible for a woman to look at the area between the navel and the knee of another woman, except in cases of necessity such as medical treatment. This does not necessarily mean that a woman is supposed to show the whole of her body except this area. Rather, it means that if a part of her body is exposed, she will not be held responsible before Allah.
In case a Muslim feels a doubt against another woman’s way of looking at her or she is afraid that such a woman may describe her body for a male, she must cover her body in her presence. Such is the case when a Muslim woman is attended by a non-Muslim female.
Allah Almighty knows best.