As a federal retiree, she has many plan options. A majority of them, probably including BCBS, will pay the Medicare premium. As other posters noted, this will mean she pays pretty much nothing for her care beyond the premium.
Under Medicare alone, she would pay the Medicare premium, and then after that, pay deductibles, copays, and for uncovered items.
With both, she only pays the federal health plan premium (after they pay the Medicare premium).
She has the most expensive federal health plan. She could switch that. Her husband's death is probably a qualifying event, or you can do that during open season for FEHB.