In a Title VII action for hostile work environment, when an employer becomes aware of harassing or violent conduct by a thord party, it has an obligation to attempt a remedy.
A wrongful termination action under the same statute presents an issue for trial where the protected activity is a refusal to deal with the harassing third party and there is a showing of direct evidence associating the refusal with the firing.
http://www.ca5.uscourts.gov/opinions/pub/17/17-60072-CV0.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.