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.
It is permissible for a Muslim to keep ties with his non-Muslim father or mother as this is an act of dutifulness to them, and this is something ordained by Allaah. Allaah Says about non-Muslim parents (what means): {…but accompany them in [this] world with appropriate kindness.} [Quran 31:15]; which means to be kind and dutiful to them.
Al-Qurtubi said: “The verse is evidence for the obligation of keeping ties with non-Muslim parents with whatever money a person can afford…” It is also confirmed that Asmaa' Bint Abu Bakr said: “My mother came to visit me while she was a polytheist at the time of the Prophet so I asked him [the Prophet ]: ''My mother came wishing that I will be kind to her as her daughter, shall I keep ties with her? He said: ''Yes, be dutiful to your mother.''
Therefore, it is permissible for you to help your mother financially every month, but if you know that your mother will spend the money in what is forbidden, like consuming intoxicants and the like, or spend extravagantly, then you should not give her money, and instead you may buy for her the food, drink, clothes, and other things she needs.
You should try your best to call your mother to Islam with wisdom and good admonishing as this is the best thing which a Muslim could do for his non-Muslim parents. Allaah might save your mother from Hellfire and you will be a reason on the Day of Judgement for her being saved from Hellfire. You should supplicate Allaah as much as possible to guide your mother to Islam and open her heart to it, because supplication is a great means of which many people are heedless. Allaah Says (what means): {Call upon Me; I will respond to you.} [Quran 40:60].
Finally, we ask Allaah to guide your mother to Islam and open her heart to faith.
For more benefit on a woman who complains about the extravagant spending of her mother, please refer to Fatwa 88664.
Allaah Knows best.