Apple ID address is essential when you contact apple support senior care advisors to recover her account Get a copy of the data associated with your Apple ID account - Apple Support
in case if you have details of her friends contact them , she might be sending mails to them the Apple ID address ( that is the mail address ) might be obtained from them , the device is lost it depends if passcode Set a passcode on iPhone - Apple Support was enabled then the person who now access it can't open the device and delete photos / videos directly from the device .
See the article If you forgot your Apple ID email address or phone number - Apple Support
Cross check if iPhone was signed in with same Apple ID and password on the same network with Mac / iPad in family , the Apple ID will be visible from them .
Note : apple support don't keep record of Apple ID address with them as users have full rights to change Apple ID and its account credentials any time Change your Apple ID - Apple Support