wow, that stinks. I think your right she didn't know what size to buy or is just trying to get something for nothing.
I'm sad to say I see a neg coming no matter what you do. There is a slight chance she has you mixed up with the other seller she bought shoes from if they are the same.
If it was me (being the sucker I am) I would tell her I'd issue her a refund minus shipping upon reciept of the shoes if they are returned in the same condition. Sadly if she goes to paypal there is a chance she'll probably get all of her money back anyway at least this way you save on the shipping.
Let us know how it goes.