Just give her the phone, wipe it and set it up for her with her own Apple ID.
If she is already on O2 she can just use her sim if it's the correct size, or she can do a sim swap.
Any O2 HighStreet Store can perform the swap in store or a person can take the SIM card home or request one via O2 Customer Services. 202 for Contract. 4445 for PAYG. 8002 for Business ->
http://www.o2.co.uk/contactusSim Swap link ->
http://swapmysim.o2.co.ukBe sure to ask for an Account Specific Blank SIM card as Contract and Pay As You Go versions hold different information.
The Service will stop on the original SIM card which indicates the swap has started. Turning the phone Off and On , or if applicable switching airplane mode On and Off ,with the new SIM card in every 20 minutes or so will typically help the connection to settle.
If she needs a new sim, look in the shop pages for either a monthly sim only or a payg sim.
If she is on a different network you need to unlock your iPhone first.
Link to unlocking here
http://community.o2.co.uk/t5/Other-Products-Services/Unlocking-an-O2-phone-to-use-a-different-SIM-ca...
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