Why can't a 2-week-old R4 SD card be reformatted?

My friend recently purchased an R4 card with an SD card and has the latest firmware/kernel versions on it. After another friend took the SD card to put games on it, she has been unable to connect the SD card to a computer or reformat it (Windows is unable to reformat it). She thinks the friend just pulled the card without first "ejecting" it. There is no lock slider and the SD card can still be read fine in her DS. Other than buying a new SD card, what can she do to fix this problem?