http://www.leginfo.ca.gov/cgi-bin/displaycode?section=lab...
All that section says is that if the guy files a claim with the labor commissioner for wrongful discharge, then the commissioner can "take assignment" of his claim, meaning they can sue the company in his name. It doesn't really say anything about whether his firing was OK or not.
A better reference on at-will employment in California is at http://business.ca.gov/StartaBusiness/AdministeringEmployees....
Edit: s/employment/discharge/ (doh)
Truth is a defense. Did they not do what she reported them as saying?
(NB: I'm not saying I approve of her behavior, but trying to paint her as legally responsible is out there.)
Sue her individually for defamation et al. and tie her up in court. Let her pay the REAL price for wielding a public, 10,000 follower strong opinion against a privately intended comment.
In 49/50 states, it isn't. So there is no leg to stand on.
Unless he was employed in Montana.