What is the ruling on one who finds out that someone is persisting in committing a sin, and he covers up for him and is content just to advise him in the hope that he will be guided aright? Is he sinning because he has not informed the relevant authorities about him?

Praise
be to Allah

It is
permissible to cover up for him if he is not one of those who take the
matter of sin lightly and he is not known to commit a lot of sins or to
commit prohibited actions. In this case, he should advise him, tell him to
fear Allah and warn him against doing it again.

But if he is a
person who regularly commits sin and evil deeds, then the one who finds out
about it will not have discharged his duty until he refers the matter to
someone who can punish him in a way that will deter him from persisting in
it.

If the sin
affects the rights of another person, such as if he sees him stealing from
someone’s house or shop, or he sees him committing adultery with the wife of
So and so, then it is not permissible for him to cover up for him, because
that is a transgression against the rights of another person, it affects
that person’s marriage and is a betrayal of another Muslim. Similarly, if he
finds out that he killed or injured a Muslim, he should not cover up for him
and cause the rights of a fellow Muslim to be neglected; rather he should
testify against him to the relevant authorities, so that matter may be dealt
with. And Allah knows best. End quote.

Shaykh ‘Abdullah
ibn Jibreen (may Allah preserve him).