I don't think this guy deserves the title whistleblower. A whistleblower leaks the crimes as they're happening in order to stop them. Not 6 years after participating in the crimes to settle a grudge.
Other than the term "informant" or "insider" I can't really think of how you would describe this type of thing. There's whistleblowers, and death bed confessions.
I suppose someone could simply be called a "tattle tale" or a "former employee", but neither really describes the act of revealing knowledge that was not publicly available.
"Leaker" might fit, but that seems to be more for someone who is a current employee, who is trying to disclose information in a secretive manner. Advocate has an equally weird meaning, too.
Perhaps “disgruntled former employee”? “Disgruntled” is a term that often comes up in mandatory security training and describes people to whom you probably do not want to share production secrets.