A man knows that his friend is doing something prohibited out of ignorance. Should this man inform his friend? This man is afraid that his friend would mock him or not believe him and that this would harm his friend as he would no longer be excused for being ignorant of the sin.

All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.

If someone sees his friend or anyone committing a sin ignorantly or intentionally, he should forbid him and tell him that he is committing a sin, otherwise he would also be sinful. Allaah Says (what means): {And let there be [arising] from you a nation inviting to [all that is] good, enjoining what is right and forbidding what is wrong, and those will be the successful.}[Quran 3:104]

The Prophet, sallallaahu ‘alayhi wa sallam, ordered the one who sees an evil to change it physically with his hand if he was able to do so. If he could not, he should change it by his tongue (by advising and/or condemning), and finally by (detesting it with) his heart, and he, sallallaahu ‘alayhi wa sallam, informed that this is the weakest level of faith. [Muslim]

If Muslims fail to do this, ignorant people would never learn and wrong deeds would not be changed.

The fact that this person is afraid that his friend would mock him or not believe him should not prevent him from saying the truth and freeing himself from liability. Moreover, his friend should not deny or mock what he says because he is ignorant of these matters, otherwise he would be among those whom Allaah The Almighty has mentioned in the following verse (which means): {Rather, they have denied that which they encompass not in knowledge and whose interpretation has not yet come to them. Thus did those before them deny. Then observe how was the end of the wrongdoers.}[Quran 10:39]

Allaah Knows best.