Re-read the post. It wasn't crystal clear, but it looks like the question is: Can the Employer charge an extra $50 if the spouse DECLINES coverage at her ER and take coverage at his ER.
In any event, the Employer cannot charge extra premium to the employee for any usage of the plan or charge premium if the employee declines coverage for dependants. The employer would run afoul of the HIPAA privacy laws for ever asking about any claim filed by an employee.
Either way, the employer is running a huge risk of a lawsuit and federal penalties.