Luggage is a curse,, I pack relatively light and still find the numerous times I hated hauling my bags up and down stairs such a hassle. Train stations and luggage equal stress and hassle, and no ,, do not count on help , we helped a much older lady ourselves after watching people just whiz past her on a set of steps.. and we are 50 and loaded with our own stuff.
Ship stuff. Really. Its hard enough getting around without worrying about your stuff. Think about this, she is somewhere in a station and wants to go in bathroom, can her stuff fit in the stall with her???? She will have no one to leave it with and so how will she manage. PACK LIGHT .

Take TGV to Aix....then get a taxi from the station into the city....worth the money for the ease
of transfer. Ship stuff...pack light as recommended by justineparis. Make it EASY...don't
complicate the trip for a first-timer.

For a five month trip I packed a 21 inch suitcase and a regular sized backpack. It was more than enough, and I could schlep it myself. Shipping probably isn't worth the money especially when she might rather spend that money buying something locally.

Thank you all. You were very helpful.

Another problem is buying train tickets ahead of time. Her plane might be late. There might be a long line in immigration. As Michaelo and StCirq suggest, allow a minimum of 2 hours. If she misses the train, her ticket is no good; she'll have to buy a new one.

From what I see, all the later trains require a change. That means getting off the train, finding the right track for her next leg, and dragging her luggage there, probably up and down some stairs. Sounds pretty daunting for a young first timer on her own with a heavy bag.

So I think shipping the luggage is an excellent idea. Just write it down to part of the expense.

then how about "urgent luggage question".

and yes, I expected something URGENT like lost passport, broke leg, etc. While the OP posted a time dependent question it wasn't URGENT In any real sense of the word.