Failed post accident drug test

We are a private owned manufacturing company in Chino, California. Recently we had an employee injured on the job. The employee sustained some injury and was treated at our local health facility. Per our company policy, we require a 9 panel urine test following any accident. The employee tested positive for marijuana following the accident. Our company policy clearly states if an employee refuses to test or tests positive, their employment will be terminated. Employees are required to sign an understanding of this policy and the employee in question did sign the agreement. The employee states that he has a prescription for "Medical Marijuana." We have no way of knowing the timeline of the employee's use of the substance. If we terminate the employee for the policy violation, what type of repercussions could we face? Since he was injured, can he retaliate if we terminate?