What is the ruling on abandoning and shunning a brother who does not pray and frequently utters words of disbelief, given that the mother is aware of this and does not side with the questioner?

One who abandons prayer must be advised, admonished, and warned of the consequences of abandoning prayer. If this does not help, then he should be boycotted if boycotting benefits him. If the one who abandons prayer utters words of disbelief, then he is an apostate from Islam, and it is not permissible to sympathize with him. Rather, he must be shunned until he returns to Islam, in accordance with the Almighty's saying: [You will not find a people who believe in Allah and the Last Day loving those who oppose Allah and His Messenger, even if they were their fathers or their sons or their brothers or their kindred]. Scholars have unanimously agreed that whoever belittles something glorified by Sharia, such as the Quran, is a disbeliever.