I'm not aware of such a device, but I had an idea. Windows 8 should have some parental control options that let you lock down application usage. You might be able to lock it down so much that she can only open her word processor, which could be Word or any number of free applications like LibreOffice.

If that sounds ok, there are plenty of inexpensive Windows laptops these days, like the HP Stream. You only mentioned a tablet, but I'd rather have a keyboard if this is only for creating documents.

Will she be able to get by without access to the internet? Won't she need it for some projects?