Again, the Apple ID being used to log in to FaceTime is unrelated to the address she can be reached at by Facetime. she can be logged in with your Apple Id, and still be Reached by FaceTime at her Email Address.
On an iPhone and an iPad, You can go to Settings->Facetime->You can be Reached by FaceTime At->
You can then Add the Email Address there.
Completely unrelated, you can change what Apple ID is logged into FaceTime there as well, and as such what list of addresses she can be reached at. Each Apple ID would have its own list of addresses.
However, if the Email is currently associated with an Apple ID, and you wish to use that Apple ID to log into FaceTime, you will need to find out its password. This again does not preclude the possibility of adding an email address to a different FaceTime Login.