Account Locked

From: How to generate a recovery key - Apple Support

Does she have a trusted device (another item of equipment on which she has logged in with this account)?

- If you forget your Apple ID password and do not know your recovery key, you can try to regain access using your trusted device protected by a passcode.

- When you generate a recovery key, you can't use account recovery ( How to use account recovery when you can’t reset your Apple ID password - Apple Support ).

- "Using a recovery key is more secure, but it means that you’re responsible for maintaining access to your trusted devices and your recovery key. If you lose both of these items, you could be locked out of your account permanently."

Read the article for more details.

Ask your mom to read the following Apple articles:

If you can't connect or sign in to iCloud

If you can't connect or sign in to iCloud - Apple Support

If you forgot your Apple ID password

If you forgot your Apple ID password - Apple Support

What to do after you change your Apple ID or password

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

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See -> Two-step verification for Apple ID

Two-step verification for Apple ID – Apple Support (UK)

scroll to & read :

👉 Where should I keep my Recovery Key?

Keep your Recovery Key in a secure place in your home, office or other location. Consider printing more than one copy, so that you can keep your key in more than one place. Your key is easier to find if you ever need it, and you have a spare copy if one is ever lost or destroyed. You shouldn't store your Recovery Key on your device or computer, because that could give an unauthorised user instant access to your key. 

If you ever need a new Recovery Key, find out how to create a new Recovery Key. 

👉 What if I lose my Recovery Key?

You can sign in to your Apple ID account page and create a new Recovery Key using your Apple ID password and one of your trusted devices.

1 Go to appleid.apple.com/uk.

2 Sign in with your password and trusted device.

3 In the Sign-In and Security section, click Recovery Key > Change Recovery Key.

Keep your new Recovery Key in a safe place. Your old Recovery Key won't work any more and you can't use it to reset your password or access your account. If you can't see Recovery Key in the Sign-In and Security section, then you are not using two-step verification. 

FYI: write down your recovery key and keep it in a safe place.

If that works, she can then use the information in the document we provided earlier to either generate a new key, or remove it altogether. I think for many people, using a key places more burden than help.