Train from Paris CDG

Not even close. From the time you walk out of the arrivals hall, it's going to take you an hour and a half to get to Gare Montparnasse. That's if you have the rer/metro ticket in your paw, know exactly how to get to the rer station, make the rer/metro transfer perfectly at St Michel, and know which way to hike the tunnel to get to the SNCF station from the metro side at Montparnase Bienvenue. You'd better figure a couple of hours to make the transfer. That's sixteen minutes of slop. If you walk out of the arrivals hall within an hour of the airplane door opening at that time of day, you might set a world record. Airplanes are always on time, too, right?

Or, somewhat less stressfully, take the train to Tours directly from the CdG SNCF station, which perhaps is what you were asking for in the first place. It goes to the station Saint-Pierre des corps, with a very short shuttle bus into Tours, which is included in the ticket. You might make the departure at 10:16 a.m. or have to wait a couple of hours more. It's hard to predict connections from intercontinental flights to SNCF trains at CdG because so many factors cannot be controlled. SNCF offers some tickets which can be changed, at a higher price. To see the airport, including maps which you can print and carry with you, check http://www.aeroportsdeparis.fr/ADP/en-GB/Passagers/Home/
For general advice on trains, better than I can give you, look at www.seat61.com and www.parisbytrain.com

Maybe she is talking about taking the TGV to Tours from CDG?

Oops! That one's doable. Out the arrivals hall, hook a left, go downstairs. Five minutes. Buy coffee, read a newspaper, powder nose. Plenty of time.

Yes, taking the train from CDG to Tours....not Montparnasse.
Thanks, Susan