If a person goes against the law, should I report him?

The holy and noble Qur'an tell us that we should combat injustice, corruption, etc, but we should also be just. Let's say that there's someone who goes against law to gain more money (even if it's a simple law and the person is kind of nice). Should I report him to the authorities? When should I be harsh and when to be forgiving? What is better: that I forgive him or that I report him? He will lose his job and he will have to work for 2 years without a salary if I do that.