Please consider the following conditions and combinations:

They are an acquaintance, a good friend, or an enemy.

You would receive a reward or you would not receive a reward.

The number of people being impacted negatively by the crime being greater or less than the number of people being impacted negatively by you telling.

EDITED You would remain anonymous as the whistleblower, or your participation would be known by the public and by the offender. [end edit]

This is in general. Please answer with consideration of the above factors or ask questions for clarification.