Someone Sins in Private: How to Advise Him
Question :
One of our neighbors does evil things in his house, but he doesn't openly do those things in front of people. Through certain means, we know what he does. Should we advise him, even though he doesn't openly do these deeds?
Answer:
What is legislated for you is to advise him secretly in a manner that is between you and him only. You should also supplicate for him to be guided. But don't backbite him and tell others because the Prophet ﷺ said:
"Whoever covers (the sins) of a Muslim, Allah will cover his sins in this world and in the Hereafter."
Source:
Ibn Baz
Fatawa Islamiyah, Vol. 7 Page 316