well, without knowing it's maintanance history it's hard to say. Don't touch the idle control. It could be your TPS Sensor, fuel filter, fuel pressure regulator, EGR valve, or it could be electronic. test the EGR valve by lifting the diaphram(sic?), if it moves freely then it's good, if it does not then it is stuck and needs to be replaced. Is it throwing any codes? If not then more than likely it's not your fuel pressure regulator. When the TPS sensor goes bad it tends to give the motor an irregular idle rather than stalling. It could also be electrical. first check all the hoses, then replace the fuel filter (it will need replacing anyway and it will give you a starting point for maintanance since she just bought the car. lastly, get a haynes or some type of manual for her car so you can trouble shoot. More than likely it's something simple though. But to really figure it out I need more info on the cars specifics and maintanance history.