If her device is on a different apple ID, then you have to chose if she wants to reset hers and set it up with your apple ID - thus losing access to things she's bought under her ID - or look for another way.
Linking the books to a singular ID is a theft prevention device. Doesn't do them much good to sell you a copy if - theoretically of course - a person can then toss that book up on the net and let everyone get at it.(not accusing and no offense, just an example) And while some other services, like I think Kindle and Amazon has the ability for a person to 'loan' their book to another account, iBooks doesn't have that.
If she's looking for cost effective ways to read books, many libraries have e-books you can check out, usually for free. The only 'catch' is that you have a specific number of weeks to get them read.