Didn't you ask this a few months ago?
The three major carriers in France are Orange, SFR and Bouygues Telecom. My wife is French so we go to France a couple of times every year. I've used Orange and Bouygues Telecom for "pay as you go" while in France. SFR use to only offer "pay as you go data" for iPhone, but that may have changed over the years. Orange and Bouygues Telecom have basically identical coverage as far as I can tell, and I am normally around Paris or Nice, France.
Orange is a little more expensive than Bouygues Telecom, but they have more stores. I've attached a copy of the Bouygues Telecom "pay as you go" rates, it's from last fall. If she picks the Formule 24/24 plan for 35 euros, she'll get a 10 euro credit for her next refill and will have unlimited voice and sms in France and 250MB of data which is good for 35 days. I have no problem with 250MB of data lasting two weeks. My wife has a habit of streaming CNN videos so 250MB of data will only last her a week, so I always need to add more to her account. WiFi hot spots are more popular in France than they are in the states, so she should set up her phone to pick up Bouygues Telecom WiFi hot spots to save on data.
She'll get 3G and 3G+ in France. She won't get LTE.