In Khazāin-ul-‘Irfān, Ṣadr-ul-Afāḍil, Sayyidunā Maulānā Muhammad Na’īmuddīn Murādābādī عَـلَيْهِ رَحۡمَةُ الـلّٰـهِ الۡـهَادِی explains the words ‘اَوۡ نِسَآئِہِنَّ’ (or the women of their religion): In a written message, Amīr-ul-Mu`minīn Sayyidunā ‘Umar Fārūq-e-A’ẓam رَضِىَ اللهُ تَعَالٰی عَـنْهُ instructed Sayyidunā Abū ‘Ubaydaĥ Bin Jarāḥ رَضِىَ اللهُ تَعَالٰی عَـنْهُ to prohibit non-Muslim women from bathing in public baths at the same time as Muslim women did. This proves that it is not permissible for a Muslim woman to expose her body in front of a non-Muslim woman.
My master A’lā Ḥaḍrat, Imām-e-Aĥl-e-Sunnat, scholar of Sharī’aĥ, guide of Ṭarīqaĥ, Ash-Shāĥ Imām Aḥmad Razā Khān عَـلَيْهِ رَحْمَةُ الـرَّحْمٰن says: The Islamic ruling is that it is as Wājib for women to observe veil with non-Muslim women as with non-Maḥram men. This means that it is not permissible for a Muslim woman to reveal any part of her hair, arms, wrists, or any part from her neck to below her ankles to a non-Muslim woman. (Fatāwā Razawiyyaĥ, vol. 23, pp. 692)
Question: Must an Islamic sister observe veil with a female sinner (Fājiraĥ)?
Answer: No. A Fāsiqaĥ is any woman who commits a major sin or repeatedly commits any minor sin, e.g. any woman who does not offer Ṣalāĥ, hurts her parents, backbites, or tells tales. Any woman who commits fornication, or any other indecent act is a Fājiraĥ as well as a Fāsiqaĥ. Observing veil with a Fāsiqaĥ is not necessary but it is cautiously ordered to observe veil with a Fājiraĥ, avoiding her company is extremely important as it could have detrimental consequences.