Employees have the legal right to work in a harassment-free environment, and employers must take corrective measures to end harassment when it comes to their attention. It doesn't matter that the harassment comes from customers or others the employer has no control over. If your organization knows about the harassment and does nothing to stop it, you're wide open to liability.
Recent case: Corrections officer Deanna Freitag complained in writing that inmates masturbated in front of her.did nothing to stop it, so she sued. Freitag presented evidence of how other prisons handled the problem. That was enough for the court to conclude that Freitag was subjected to a sexually hostile environment. (Freitag v. Ayers, et al., No. 03-16702, 9th Cir., 2006)