If she is on your account and switches to her own account - she will have them on her Kindle until they are delete, then they are gone and she won't have access to them again. But this form of sharing books is considered illegal since you are the one who ones the books.
Giving away or selling a Kindle with content is considered against Amazon's terms of service. If you share an account and subsequently decide not to, I think an argument can be made that the sharer who leaves can take the books he/she actually purchased when they move to a new account -- subject to the conditions that if she deletes them without making her own back up they're gone for good. Pretty sure it's not been tested, and I'm not sure if this specific scenario has even been addressed by Amazon. Note that they don't currently have any way to 'split' an account, which is why I think they'd not really get bent out of shape about something like this. . . . .
That said, though I share my account with my brother, I'm not sure I'd be willing to share with my son's girlfriend. Probably not even a fiancee or spouse. . . . . . .that's possibly based as much on some of my content as anything. BUT, in terms of what happens to shared books if they break up, well, I think that's a valid thing to consider. Haven't seen a Kindle dispute on Judge Judy yet and we'd hate for you to be the first!
I think if it was me, I'd lean toward letting her have her own account. . . when I feel like giving her a little something, I could always send a gift certificate. . .maybe even with the name of a book I think she might like. . . .if she doesn't want to feel beholden, make it small amounts as 'thank yous' for any little thing. . . . .remember there are a TON of free and very cheap books available. . . . .And when she's better off, she can do the same for you if she wishes!
