Keep reading your own link. Duress includes "retribution" not limited to physical violence.
" (b) As used in this section, "duress" means a direct or implied
threat of force, violence, danger, or retribution sufficient to
coerce a reasonable person of ordinary susceptibilities to perform an
act which otherwise would not have been performed, or acquiesce in
an act to which one otherwise would not have submitted. The total
circumstances, including the age of the victim, and his or her
relationship to the defendant, are factors to consider in appraising
the existence of duress."
The definition of rape in California says nothing about "coercion":
http://www.leginfo.ca.gov/cgi-bin/displaycode?section=pen&gr...
It does mention "duress" which implies a threat of violence. If your friend's boss was not threatening violence then it's wrong to call it rape.