Go to your Paypal refund. You may have to look in the Archived items, but you can narrow down the dates to search.
This will give you the seller's email.
Use Send Money to send the payment to your seller. Include the eBay number and the PP refund number for her books.
You can add a Response to feedback left by you or by the seller indefinitely. This does not affect her fb percentage, but does show up when people look at her fb pages, so it is a nice thing to do.