I too, feel they need a car but there are many options

We have done  one-way returns 2 times in France.  The one-way drop off fee were not outrageous. The fees at that time were not outrageous,  like about $50.    totally acceptable when you consider the time saved. 

One trip was renting from Toulousse (at local location, not at the airport - they actually waited for our arrival as we were the ONLY renter from out of country to pick up car at that location in the past 12 months...  On that trip we drove all the way to Basque Country.  Then went to Bordeaux,  up the Loire Valley where we spent 6 days?  went to Chartres,  then Fontainebleu,  returned the car at Paris (the location we returned to was extremely easy - a straight drive from d'Ialie to Place d'Italie then to Saint Sulpice church underground garage.  But we could not find the Avis sign on WHICH entrance to go down the garage so we parked next to a pedestrian entrance,  walked the stairs dow to the garage,  found the Avis office almost all the way across the garage.   The office asked where the car was parked,  we said it was above on the street.  They sent a technician to lead us go up the ground - when we emerged on the street level,  the car was right in front of us.   It was a funny scene.)  I believe Hertz has a location also very easy to get to, at the Touileries right next to the Louvre Carrousel. 

Another trip was renting at Nice,  went thru Provence and visited Gorges,  to Lyon then Dijon,  returned the car at Paris (this time at another location - even though it was at the outer ring at Bolognes,  getting there was not easy at all even after our hotel printed out a colorful Google map and showed us how to...) 

Key is to pick a location in Paris that is truly easy to drive to.

We also have trips in France that we picked up our rentals by the railroad stations.  This is a very common set up in Europe.  Railway stations and airport locations command a location surcharge due to the convenience. 

For example, our trip to Normandy and Bretagne,  due to it was after Mid Sept, there was no TGV from CDG going to Caen, our original plan.  You would need to take the train from Montparnasse in the city...  So we changed our plan to go to Rennes, and started our trip from there. 

I think the OP can use the Railway station approach - first figure out which city as starting / end point of the trip outside Paris.  Then check the rental car options.