Apple ID

See Get a verification code and sign in with two-factor authentication https://support.apple.com/en-us/HT204974  Below is an excerpt, but read the whole document. See the bold italics below.

“Whenever you sign in with your Apple ID on a new device or browser, you'll confirm your identity with your password plus a six-digit verification code. There are a few ways you can get a verification code. You can use the code displayed on your trusted device, get a text or phone call, or generate a code from your trusted device [see the additional information in the article].” A trusted device is an iPhone, iPad, or iPod Touch with iOS 9 and later or a Mac with OS X El Capitan and later that you've already signed into using two-factor authentication.

You may be able to get the code texted to the phone number associated with the Apple ID or generate one from another iOS device logged into the same Apple ID.

See Get a verification code and sign in with two-factor authentication – Apple Support

I would try getting Apple telephone support  for help in solving your problem using this link https://getsupport.apple.com/?caller=kbase&PGF=PGF63005&category_id=SC0245&symptom_id=23362 Be patient sometimes Apple support is closed to immediate phone support.

Where is the number received?

Thank you. On what device?

When your mum enters the code, is she seeing a specific error message?