The employer is most likely not liable. An employer is not  its employee's insurer; it is only responsible  for their injuries generally if they were hurt in the course of employment and/or due to employee negligence (carelessness). "Messing around" or "horse play" with a pressure washer is NOT part of the employee's job descriptions--it's not what they were hired or expected or told to do. Therefore, since what they were doing was not part of their jobs, the employer is most likely not responsible.