Use If you forgot your Apple ID password https://support.apple.com/en-us/HT201487 to recovery this credential.
If you cannot recover your Apple ID password and have not been able to answer your security questions successfully), then see:
Security and your Apple ID https://support.apple.com/en-us/HT201303
This document says “without proof of your identity via security questions and other carefully selected criteria, Apple Support can't help you reset a password or perform any other actions on your account.”
So, per Apple, she will not be able to use her iPhone unless there are extenuating circumstances. Still you can try to solve her problem using the support options below. Be ready to provide any additional information to Apple you think may be pertinent to solving her problem (e.g., her original purchase receipt with the serial number of the iPone on it).
Get a call from Apple support to help solve your problem using this link https://getsupport.apple.com/?caller=kbase&PGF=PGF63005&category_id=SC0245&symptom_id=23362
Apple ID Support https://support.apple.com/apple-id
Apple ID account security for country-specific and region-specific help https://support.apple.com/en-us/HT204169
Get Support: We’re here to help https://getsupport.apple.com/. Go through the options to find the best support option.