Apple ID lost

She can check her Apple ID via her email box, see this Apple article: If you forgot your Apple ID

If you forgot your Apple ID email address or phone number - Apple Support                  

OR see -> Having trouble signing in?

https://iforgot.apple.com/password/verify/appleid

If you've forgotten your Apple ID email address or phone number

If you've forgotten your Apple ID email address or phone number – Apple Support (UK)

Then see this Apple article: What to do after you change your Apple ID or password

What to do after you change your Apple ID or password - Apple Support

Try the following:

▷ Find your Apple ID.

▷ Forgotten Apple ID Password.

Need more help?

• Call Apple Support at 1-800-APLCARE.
• Contact Apple Support.
• Chat using Messages.