Unlawful harassment may be committed by third parties in the workplace, such as customers, sales representatives, subcontractors, repair workers or independent contractors. Employers may be liable for the sexual harassment of an employee by a non-employee under certain circumstances:

• the employer knows or should have known of the conduct;
• the employer has some control over the situation or is otherwise legally responsible for the non-employee’s conduct; and
• the employer fails to take immediate and appropriate corrective action.

Depending on the circumstances, such action might be relatively simple—for example, switching table assignments. In this case, the manager should arrange to have a waiter finish serving that table and make whatever arrangement might be necessary so that Maribeth would not be financially or otherwise harmed by the substitution (for instance, by losing the amount of a tip she could have earned).

Source: CCH HR How-to: Harassment Prevention