I also believe the employer has a right to terminate their position with the company if they will not comply with the terms of their employment.
Consider what most people would say if you hired a Muslim and they refused to honor customers' requests to purchase liquor and pork. You would be well within your rights to dismiss them from their job.
He/she doesn't have to do it...but the store manager better find someone else who can. What if a Jehovah's Witness Pharmacist refused to sell anyone aspirin?