I don't have enough information to know if your friend's Medicare is truly primary or secondary.
If someone has an employer group health plan, Medicare is secondary to that plan for the first 30 months the person is eligible for Medicare. If the person's first treatment is a transplant, he/she would be eligible for Medicare the month of the transplant and in certain cases a couple of month before. If the person's first treatment is dialysis, when Medicare eligibility starts depends on whether the patient chooses to do in-center dialysis or not. If so, Medicare eligibility starts the third full month of dialysis. If the person starts a home training program before the first day of the third full month of dialysis, Medicare eligibility starts the first of the month that dialysis started.
If her employer plan is her primary payer because she hasn't been eligible for Medicare for 30 months yet, that plan would be responsible for paying for any covered drugs as long as she goes to a pharmacy on her plan. If that pharmacy accepts Medicare, it could bill Medicare second.
If Medicare is primary because she's been eligible for Medicare for 30 months or longer and she has Part B, then I'd advise that she get her immunosuppressants from a specialty pharmacy that knows how to bill Medicare correctly for transplant drugs. Patients report that they have few problems with specialty pharmacies. Her transplant program should be able to tell your friend where most of their patients get their drugs under Medicare Part B.
If Medicare is primary, her employer plan would probably not want to be the primary payer for her immunosuppressants so she probably has no choice for which coverage to use for the transplant drugs. I'd recommend that she call the state insurance department and talk with a health insurance counselor to find out if state regulations allow employer group health plans to refuse to pay secondary to Medicare for Part B covered drugs.