I can confirm that the American Express for Target prepaid card works on Mog Station for Final Fantasy XIV. This card is obtainable at select Target stores and costs $3 to load, with no monthly fees.

I have used them to purchase Crysta (essentially a deposit to your Square Enix Account balance), then use Crysta to prepay for my subscription. I have not used them for automatic subscriptions, but the Crysta balance can be used for all subscription terms and costs the same. If your friend has trouble with using a card with the monthly subscription, she may want to try using it to add Crysta.

I have used the Serve from American Express card with other game publishers in the past. The Bluebird card is on the same platform as Serve, so using it should have similar results.

Please note that these kind of prepaid cards are different than gift cards. They often look the same but gift cards do not require identification to activate and can't be reloaded.