Yes, if this is a violation of your substance abuse policy. An employee should not be disciplined or fired for seeking treatment, but can still be held to the same standards as other employees. If you do not allow employees to be intoxicated at work, an employee in a recovery program faces the same consequences as other employees for violating that policy. If other employees are disciplined or fired for being intoxicated at work, an employee in recovery could also be disciplined or fired for that reason.