The difference between a MCR supplemental and secondary is that MCR supplemental plans simply "supplement" MCR, meaning that they do not cover things not covered by Medicare, they pick up the balance after Medicare. Secondary plans follow their own policy, so if there is a service not covered by Medicare it may still be covered by the secondary.
The client you are referring too has an employer sponsored plan that writes their own rules. Apparently they have specified that if an employee (or retiree) becomes eligible for MCR then they become a supplemental plan. I bet if your client looks in him employee handbook it will state that. It's something that people don't pay attention to until it affects them.
Hope that helps!