Yuitsu you are under no obligation to give her a partial under any circumstances, especially since she altered the item. Sure, she can file an item not as described case and ebay will force her to return it but since she altered it you can fight that.
I would politely explain to her that Ebay policy states that:
Policy overview
You must return an item in the same condition as it was received. You must also include, in the same condition as it was received:
All packaging such as boxes, bags, and tags
Accessories such as cords and remotes
Certificates, manuals, and warranty cards
"Free gifts" or bonus items
All items included in a bundle
Additional requirements for items in specific categories are outlined below.
Make sure you follow these guidelines. If you don't, a seller isn't required to accept your return. You may also be subject to a range of actions, including limits of your buying and selling privileges and suspension of your account.