I found this on the U.S. Department of labor web site. I don't think it is legal at all. Either My employer has to hire another person or make the part timer into full time.
Two or More
The phrase “two or more other employees” means two full-time employees or their equivalent. For example, one full-time and two half-time employees are equivalent to two full-time employees. The supervision can be distributed among two, three or more employees, but each such employee must customarily and regularly direct the work of two or more other full-time employees or the equivalent. For example, a department with five full-time nonexempt workers may have up to two exempt supervisors if each supervisor directs the work of two of those workers.
U.S. Department of Labor -- WHD: FairPay Fact Sheets by Exemption - Executive Employees