That really is a bit of a cheek. I'd probably get it back then when when she asked to borrow it say the dog bit it and you threw it out. Hopefully by the time you need it again, then shes already bought her own, and you can have your own again.
Or just start dropping strong hints that she needs to get her own and when you said she could borrow it, you never intended for it to live at her house and if she needs it that often, then she really needs to sort something herself. Easy to say from this end of the computer though.