There are a number of options. I am in Paris several times a year and I have tried them all!!! I am pretty adventurous and am comfortable trying everything. My thoughts....
Taxis are great when there are two of you and you can share the $$. I usually land at CDG alone, though, and I often find myself still debating what method to use as I pass Customs!
The best thing about a taxi is the fact that it will get you right to the door of your hotel. If you are tired and jet lagged it is great. It is, for me, not so great when the traffic is going at a snail's pace and you are watching the meter tick away. Tick...tick...tick...there goes my FSH money!!
I have taken the train (the RER). It is the seediest option--the train from the airport is not exactly elegant and you do have to consider pickpockets. Utterly jet-lagged tourists make easy pickings! My problem--I either have to change trains to get a metro to my hotel (then deal with stairs), or get off at a major stop like St. Michel...head up to the street, and grab a taxi from there. One benefit--if you have little time, it is pretty fast. Love whipping into the city faster than the traffic. Not for the faint of heart or the weak or those who love luxury, nor for those who cannot handle dragging their luggage through awkward passages.
A middle ground that I use most often is the Air France bus. Clean, restful, safer than the train. In bad traffic it will at least take the same time to the core of the city as a taxi. Check out where it stops. I usually grab the Air France bus, get my suitcases, and grab a taxi to the door of my hotel. The bus loads your bigger suitcases underneath and you carry your carryon on board. The only awkward part, depending on which stop you choose, can be finding a taxi. I do this the most because it is safer than the train but cheaper than a taxi.
One note--when I am on the train, or even if I take the Air France bus and know I will run a block to a taxi, I have a suitcase system. Keep in mind--I travel with one carryon, a purse (usually in a Longchamp bag), and two of the most massive suitcases made. I arrive with the two biggies empty and leave with them packed with stuff. When I land and go to baggage, I put my carryon in one empty suitcase and my purse in another. Minimizes how many objects I would be awkwardly juggling, and minimizes the risk of getting pickpocketed. My hunch is that pickpockets would be less inclined to wrestle a massive suitcase out of my grasp...
Hope this helps. You know your friend's personality, budget, and stress risk the best!
Oh--I always arrange a shuttle to get from my hotel back to the airport. Much cheaper than a taxi and reliable.
From my experience with the London Subway, I thought it would be difficult for her. Looks like the Metro is out till after the luggage has been dropped off!
