Relevant Rule

An employer has a legal obligation to take action to prevent and correct sexual harassment even when the harassment is being done by an outsider
to the organization, such as a vendor, supplier, customer, or visitor.

In such cases, the employer may even have the duty to bar or ban the vendor, supplier, customer or visitor from the employer's premises.

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http://www.eeoc.gov/types/sexual_harassment.html

See the sentence: "The harasser can be the victim's supervisor, an
agent of the employer, a supervisor in another area, a co-worker, or a
non-employee."