When I go to her account page and edit the AppleID, it seems her icloud address is not allowed. There's a drop-down option to make the IDs the same, and this list contains only the email address she no longer owns, or her icloud address. Selecting the icloud one and pressing submit results in the page refreshing and claiming the details have been change, but they haven't.
What's the solution?
iMac, OS X Mavericks (10.9), 3.4HHz Intel i7, 256GB SSD, 8GB RAM