I don't think it is a violation:

"Employers may require that an employee’s request for leave due to a serious health condition affecting the employee or a covered family member be supported by a certification from a health care provider. An employer may require second or third medical opinions (at the employer's expense) and periodic recertification of a serious health condition. An employer may use a health care provider, a human resource professional, a leave administrator, or a management official – but not the employee’s direct supervisor – to authenticate or clarify a medical certification of a serious health condition."

From: http://www.dol.gov/whd/regs/compliance/whdfs28.htm