As others said. Just in case she does not understand, explain to her that you will refund, and she has to process the payment again. All she needs to do as she goes through the checkout, to click on the EDIT link next to her address and give the address she wants it to go to. You could also mention to check next time she makes a payment, that it did not get changed permanently. You can have several gift addresses recorded there.
I once sold a doll and her eyelash fell off. The buyer was happy to fix it herself if I send her some lashes, and as i did not have any, i bought some and had them shipped directly to her, registering her address as one gift address with PP. Except then I bought something small and did not notice her address is there as the postal address. Did not realize until weeks later when asking the seller why is my item sent within the same city taking so long and she said, we are not in the same city, you are in Queensland, and I said , no I am not… LOL well my doll buyer got an extra little present.