Your Mother - or you, on her behalf, if you are helping her - should find this guidance to be helpful in resolving the problem...
If the device is Activation Locked, this help page will provide most of the information that you’ll need:
https://support.apple.com/en-us/HT201365
If you have forgotten the AppleID or associated password, these can be recovered here:
https://iforgot.apple.com
To recover the credentials, you’ll need access to Any one of the following:
- The primary email address mailbox that corresponds with the AppleID
- Any of the secondary/recovery email addresses that should be configured for the AppleID account
- Any of the trusted telephone numbers (fixed line or Cellular/Mobile) that are associated with the AppleID
Unless you AppleID account has not been fully/correctly configured, or has been seriously neglected, recovery of the AppleID and password should not be difficult.
More information about recovery of your AppleID:
https://support.apple.com/en-us/HT201354
More information about recovery of your AppleID password:
https://support.apple.com/en-us/HT201487
When you recover access to the AppleID credentials, you would be well advised to log-in to the AppleID account from a web browser - and verify/update any email addresses and trusted telephone numbers:
https://appleid.apple.com