He is an informer. When you realise that the company that you are working for is into all kinds illegal activities and you decide to inform the police or the public about them, then you become a whistleblower. It is also possible to write the word as “whistle-blower”.
Here are a few examples. *If you expect me to be the whistle-blower Mahesh, you can forget it. *Some whistleblower put Chandru behind bars for a couple of years.