If she is in a country where
eGift cards are available, then she could purchase one with her remaining store credit and send it to you.
(If not, then she might be able to change her address to a country where eGifts are available to make the purchase, but I'm not sure exactly how that works, there would probably be a little loss on the currency exchange. Kobo sells eGift cards in New Zealand, the minimum is $NZ10 or about $US8.50).