Aromatase Inhibitors

Does anyone know the answer to this? If a patient has already failed one aromatase inhibitor, will she fail all aromatase inhibitors? My mother's onc is giving her a choice of going on Femara or Faslodex. But she's already failed Arimidex, so I didn't understand why Femara would be an option since both drugs are AI's. Any input would be appreciated.

Thanks,

Stephanie, daughter to Marie