See If you forgot the answers to your Apple ID security questions https://support.apple.com/en-us/HT201485
If you are not able to answer these questions satisfactorily, see
Security and your Apple ID https://support.apple.com/en-us/HT201303says “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, given that you can't deal satisfactorily with your security questions, we won't be able to help you with your problem unless you have an extenuating circumstance (e.g., you have the receipt for the purchase of the pertinent device with its serial number on it or you have a court order instructing Apple to assist in the provision of access to a decedent’s information from the deceased person's accounts). If you have an extenuating circumstance, even one not mentioned above, then use the support options below:
If pertinent, see How to request access to a deceased family member's Apple accounts https://support.apple.com/en-us/HT208510).
Get a call from Apple support to help solve her 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.