Wa `alaykum As-Salamu wa Rahmatullahi wa Barakatuh

It depends on the type of person and the act of haram itself. If a person was in private and the sin is between him and God like drinking wine or committing zina with a female in private you have no right to expose him. You keep it to yourself and only if he knew that you have knowledge and he came to ask you for advice you advise him directly.

If he did not know you knew, the advice should be in general matter as a reminder.

Only when someone takes his sins to public you can speak of him.

Now if the sin is causing public harm like theft or killing someone and police is looking for him and asked public to help then yes you are helping protect public from the harm this person is causing.

Almighty Allah knows best.