3.5mm is cheapest.
You could get a Bluetooth audio receiver
You could get an iPad dock with audio out.
Or you could get an airport express.
Does she want to use the iPad while she listens? If so I would recommend the Bluetooth or airport express as they are cable free options.
A cable like that is by far he cheapest unless the stereo is already AirPlay compatible.
Airport express and bluetooth will plug in via cable. It jus the connection between the iPad and the bluetooth/airport express is wireless