Cell phone - yes they do work but it's incredibly expensive to call across the big pond. If you have a US issued cell phone, even calls within France will go to the US first and then to France and be very expensive. She would be better off buying a cheap one there...
Currency is always cheaper in the opposite country - Euros are cheaper in the US and Dollars are cheaper in France. Have her exchange the money in the US airport as she leaves and change leftovers back in the Paris airport as she comes home. Have her keep the money in several places, some in the pocket, some in the purse, some in something to hang on her neck under her shirt...
Prepaid cards - make sure it will work there. Has to be Visa or Mastercard issued - no other brand on it. Travellrs checks can be bought at your bank and work too - but she will have to cash them at the Hotel primarily, and I am not sure how easy it is in France to cash them. Countries vary.
Some more info here, if you scroll down a bit:
http://www.french-at-a-touch.com/New..._to_france.htm