Depending on what is not remembered they may be ways around this.
But you have not told us what they don't remember?
If she forgot her Apple ID, she can try to sue http://iforgot.apple.com and try to recover her Apple ID and password.
If she forgot the passcode to unlock the screen, then the iPhone can be restored without it.
If she forgot something else you need to tell us what it is she forgot.
Saying she forgot all the information entered when it was setup, is like saying they have amnesia and don't remember half their life.
In all honesty, this sounds more like a phone that was found, or bought from someone that was not the owner, than anything else.