This thread might be of interest. http://community.giffgaff.com/t5/Help-Support/Missing-points-on-referral-SIM-s/td-p/16997216
Has your daughter had a giffgaff account previously? Could she possibly have reused someone else's abandoned username?
Try searching for her account on this forum, to see if she appears to have posted anything before she joined? And check whether her join date is show correctly?
The community join date does not tell you everything - it is quite possible that someone may have activated a SIM but never joined the community.
It says in that thread that giffgaff will sometimes give the £5 joining bonus even though it should not be given to a returning member, but that the recruiter would be shown as the original recruiter and not you.
If it had been a SIM swap, she would not have received the £5 joining bonus.
Please let us know if you find a solution to this mystery - I've seen posts like this occasionally and never known whether there was a simple explanation, or whether it was indeed a system glitch.