Was the phone locked to a carrier, or was it purchased SIM free from Apple? If it's SIM free, you will need a microsim from Orange. I'm not sure about PAYG on the iPhone, but assuming you can do it that way, all she has to do is download the latest version of iTunes and sync the phone by connecting it to the Computer using the provided USB cable.
Is the phone running IOS4.x or has it been updated to IOS5?
If you want detailed information on setting it up, post back.