Is it obligatory to say that God will forgive a person who has committed a sin, or should one remind them that this sin carries a punishment in the Fire or the grave?

It is not permissible to definitively assert forgiveness or lack thereof for a specific individual, as the matter rests solely with Allah. It is incumbent upon anyone who witnesses a reprehensible act to forbid it, clarify its ugliness, remind the person committing it of the warnings associated with it, cautioning them against its consequences, and expressing fear that the warnings might befall them.