What should we do if one of our neighbors repeatedly insults Allah and His Messenger with malicious words, knowing that informing the authorities would be to our detriment?

Slandering Allah and His Messenger is disbelief (kufr), and changing this evil is obligatory if one has the ability. Whoever witnesses such an evil must strive to change it with his hand; if he cannot, then with his tongue; and if not, then with his heart. Slandering Allah and His Messenger is among the greatest evils and is disbelief in Allah, as explicitly stated by Shaykh al-Islam and Ibn Qudamah. This dangerous phenomenon necessitates that the wise people of the Ummah strive to convince people of its seriousness and that it nullifies Islam, and that its perpetrator deserves to be killed (this punishment is only executed by the ruler). Engage this man in a calm discussion and convince him to abandon this habit, and provide him with materials that clarify the danger of apostasy and its punishment.