The short answer to this question is yes. In fact, the EEOC announced in late October, 2013 that an employer is paying $30,000.00 to settle such a sexual harassment suit. In that suit, a female receptionist was allegedly sexually harassed by a male client at her place of employment.
That employee then allegedly complained to her supervisor about the sexual harassment, but no action was taken by the employer to stop the alleged harassment. The EEOC filed suit against the employer on behalf of the employee.
The lesson to be taken from this is a reminder that an employer can be liable for sexual harassment committed by a non-employee such as a client or a vendor if the employer knew about the conduct and failed to take appropriate corrective action.