Discussions on using purchases from multiple AppleIDs in one iTunes library - https://discussions.apple.com/thread/4287298?answerId=19543804022#19543804022
As I mentioned earlier, the main time when this becomes an issue is if you need to do something involving associating a computer with a particular AppleID. Careful management of your collection should minimize this situation.
iTunes Store: Associating a device or computer to your Apple ID - http://support.apple.com/kb/HT4627 - In connection with, "When you turn on iTunes Match or Automatic Downloads, or when you download past purchases on an iOS device or computer, that device or computer becomes associated with your Apple ID." "Your Apple ID can have up to 10 devices and computers (combined) associated with it. Each computer must also be authorized using the same Apple ID. Once a device or computer is associated with your Apple ID, you cannot associate that device or computer with another Apple ID for 90 days." - Additionally instructions for "Removing an associated device or computer from an Apple ID"
So the first account is really "yours" and you are setting her up with her own account? It helps to know this because if both are "hers" then there isn't an issue with her having full access to both accounts. If in 10 years she moves 2000 miles away and she is pretty much independent then you may not want her to have full access to your AppleID just so she can authorize a device.