Praise be to Allah.
There is no doubt that the story you have told us is very sad and moving, especially when we see someone suffering because the person who is closest to him is lacking in terms of religion. But ‘Allaah (Alone) is Sufficient for us, and He is the Best Disposer of affairs for us.’ [Aal- ‘Imraan 3:173 – interpretation of the meaning] ‘Truly, to Allaah we belong and truly, to Him we shall return.’ [al-Baqarah 2:156 – interpretation of the meaning].
This brother should remember that honouring one’s parents is one of the most important duties that human beings have towards one another, because Allaah says (interpretation of the meaning): “And We have enjoined on man (to be dutiful and good) to his parents. His mother bore him in weakness and hardship upon weakness and hardship, and his weaning is in two years – give thanks to Me and to your parents, - unto Me is the final destination. But if they (both) strive with you to make you join in worship with Me others of which you have no knowledge, then obey them not, but behave with them in the world kindly…” [Luqmaan 31:14-15]. These mushrik parents were doing their utmost to make their son associate partners with Allaah, yet in spite of that Allaah commanded the son to maintain a relationship with them and behave kindly with them in this world. So you are also obliged to treat your mother well, advise her to give up her bad behaviour, explain to her how sinful her deeds are and what punishment awaits her if she does not stop. If she responds, then praise be to Allaah. If she does not, then keep away from her in a good way; do not mix with her in ways that will adversely affect your religion, but do not upset her either. Behave with her in the world kindly, and continue to advise her from time to time. Your keeping away from her will not be a sin, if you do it for the sake of Allaah and in opposition to something wrong.
(See the response of Shaykh Muhammad ibn ‘Uthaymeen in Fataawaa Islamiyyah, 4/196; al-Lajnah al-Daa’imah fi Fataawaa Islamiyyah (Standing Committee on Islamic Fatwas), 4/204; Shaykh ‘Abd-Allaah ibn Jibreen in Fataawaa al-Mar’ah al-Muslimah, 2/957)
In conclusion therefore, if your staying with her will benefit her by increasing her religious commitment and eemaan, her adherence to Islamic duties and her avoidance of haraam deeds – or at least reduce her involvement in such activities – because she feels that she is being watched, for example, or because this will keep bad people away from her, and this will not cause you any harm at the same time, then stay with her in the hope of earning reward for whatever efforts you make, and Allaah will reward you for your patience. If you have already tried your utmost, to no avail, and have found that you are not making progress in any of the ways referred to above, and staying with her will have an adverse effect on your own religious commitment or reputation, then there is no sin on you if you keep away from her, as stated above, so long as you continue to check on her from time to time, to pay attention to her needs and to advise her. We ask Allaah to bless you with patience and reward you for your efforts, for He is the best of supporters and helpers.