"mens rea
[menz ˈrēə]
NOUN
law
the intention or knowledge of wrongdoing that constitutes part of a crime, as opposed to the action or conduct of the accused.Compare with
actus reus.
"a mistaken belief in consent meant that the defendant lacked mens rea""
The whistleblower is not part of the crime. And mens rea has nothing to do with not being able to fire the whistleblower.
The False Claims Act and the Sarbanes-Oxley Act are what protects whistleblowers from being fired for informing on illegal acts by companies.