How can a mother who uses shamah during the daytime in Ramadan be convinced that her action is impermissible, and what are the consequences for her son if he allows her to continue despite his numerous attempts?

Using "shamah" (chewing tobacco) is forbidden and invalidates the fast. Honoring and respecting one's parents is obligatory. Allah Almighty has commanded kindness towards them and prohibited anything that might harm them. Advising your mother should be done gently, kindly, and respectfully. If she does not respond, you can seek help from some righteous women to advise her without her knowing the source of the advice. If she still does not respond after that, continue to associate with her kindly and pray for her guidance.