Yes she can she will need a PAC code from O2 and then just complete the form details I leave below.
This link will take you too the form to fill out with your pac code
Also here's a link to a video to show you how to bring your old number.
And a link to answer any questions you may have.
Hope this helps you.
Hiya @sumab03. Yes. She will need to ask O2 for a pac code.
See this link for all the info you need.
There is also a video
Here from the kb
Hey @sumab03 once she has activated her giffgaff sim card and it's up and running. She will need to request a PAC CODE from 02 and then once got the Pac Code will need to use this to transfer the number over to giffgaff.
If you need help on how to input the Pac Code then ask the question here again once your friend is all set up on giffgaff and got her Pac Code.