I believe the Code your asking for is to Unblock the Sim Card as they've blocked the Sim Card? Correct me if I'm wrong. However your friend will have to call o2 to explain to them it's code for the Sim Card so they Unblock it. This is the PUK Code. NUK is the one if you wanting to unlock a device to use on any network.
Guide: Coronavirus Community Help and Support -https://community.o2.co.uk/t5/Other-Products-and-Services/Coronavirus-Covid-19-Community-Help-and-Su...
When you call, you will be asked for the reason you are calling.
Stay silent for 15 seconds and you will be taken to the keypad options.
Press 2 for Everything Else
Press 5 for More Options
Press 7 for Anything Else which will put you though to someone.
You can also try saying "Upgrade" or "Fraud" to be connected to an advisor.
8.00am is the best time to call if you can.
Because you're not buying anything, O2 doesn't consider your call important, so use the Upgrade, Lost/Stolen, or Fraud option if you have to, to get through. Otherwise you'll just be disconnected.