I've worked on the same model iMac and it's as easy as plug 'n play 'cause she's running os X. Please note that there's an Airport Adapter required, so you can't just plug the airport card into the iMac slot. So it's:
The airport slot is located at the bottom of the computer, you can open the small 'door' giving acces to the RAM and airport slot(s) using a coin (no tools required). Next just plug the adapter in the iMac, the Airport Card in the adapter and connect the little wire (= antenna) to the Airport Card.
By the way if she still has the manual, it's probably also in there. Good luck!
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