From the website it sounds like she should fill in a claim form and send it in probably with a letter to explain the circumstances - here is what is on the website:-
Child Benefit can only be backdated for up to three months from the date your claim is received in the Child Benefit Office, so it's best to make your claim straight away to avoid losing money.
You should claim Child Benefit as soon as:
your child is born
a child or young person comes to live with you that you are responsible for
you adopt a child who is living with you
you start to contribute to the cost of looking after a child that you are responsible for, unless the person the child lives with is already getting Child Benefit for them. The amount you contribute must be the same as, or more than, the weekly amount of Child Benefit you get for that child.
You can claim in any of the following ways:
(The forms below are in Portable Document Format (PDF). To be able to view and print out a PDF document you must have a PDF reader installed on your computer. Adobe reader is provided for download)
complete a Child Benefit claim form on screen and print it out
print out a blank Child Benefit claim form (PDF 117K) and fill it in
if your child has just been born - fill in the Child Benefit claim form that was in the 'Bounty Pack' that you may have been given.
Send the completed form to the Child Benefit Office with the child?s original birth or adoption certificate. Before completing the claim form you must read the Child Benefit claim form notes (PDF 101K).
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Documents to accompany your claim
You will need to send us the child?s original birth certificate, or the birth certificate you get after adoption, or the adoption certificate. You do not need to send us the certificates for any children that you (or someone else) have received Child Benefit for in the past.
Please note: We cannot accept photocopies of certificates.
If you cannot send us the certificate now, send us the claim form anyway. We will ask you to send the certificate later.