All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
Deserting one of your parents or both of them is contrary to what Allaah, the Almighty, has commanded in His Book. Indeed Allaah orders the Muslim to be kind and dutiful to his parents and keep good company with them even if they are non-Muslims. Allaah says (what means): {But if they endeavor to make you associate with Me that of which you have no knowledge, do not obey them but accompany them in [this] world with appropriate kindness.}[Quran 31:15]
So, Allaah forbids obeying them in committing sins but orders keeping good company with them. There is no doubt that deserting them is contrary to being kind to them, which Allaah has ordererd.
Therefore, you should fear Allaah and not desert your mother. Instead, you should keep advising her and reminding her of Allaah in a soft and gentle manner, and with what suits her status as a mother.
For more benefit on how to deal with a sinful mother, please refer to Fatwa 83028, and to Fatwa 84235 on the ruling of abandoning the prayer, and to Fataawa 84942 and 84624 on being kind and dutiful to one's parents.
Allaah Knows best.