Can an employer require an employee to complete a rehab program?

An employee has a disability – they have disclosed their addiction. To accommodate them, we are allowing a flexible work schedules and for them to receive treatment. In addition to this, can I state in the accommodation that if they attend work intoxicated they will be sent home and will need to complete a rehab program and provide proof of completion in order to return to work? Can we as the employers require this without paying for the treatment? If we can can we require a treatment for a minimum number of days?  
 
Thank you