Employer not liable for workplace defamation since it wasn't in the employer's interests, wasn't condoned by the employer, and it was outside the scope of the tortfeasor's employment; there are literally millions of workplace interactions.
http://www.ca4.uscourts.gov/opinions/171890.P.pdf
Since 2011. An occasional commonplacing of the U.S. Circuit Courts of Appeal.
Neither an authoritative statement of the law nor legal advice. Quick work.