Does she already have a National Insurance number?
- If she grew up in the UK, she would have been automatically allocated one when she turned 16.
- If she didn't grow up in the UK, and doesn't have an NI number yet, then she will need to apply for one (same as you will need to, if you wish to work in the UK).
You/she can start working in the UK without an NI number, but you won't be taxed at the correct rate until you have one, and also most employers will ask for your NI number when you apply/start working, so it's best to get it sorted as soon as you can after you have moved to the UK (as it can take up to a few weeks for it to be issued)
See here for how to apply for an NI number:
https://www.gov.uk/apply-national-insurance-number