Frieda, of course, offers inspired choices of areas.
I enjoy the Dordogne anytime of year, but your September dates make me think
of how wonderful the vineyards are then: just before, during, or after
harvest. Alot of intensive work going on, of course, but a festive air as
the results of a year of growth become known. Either the wine route south of
Strasbourg, the vineyards of Provence (where the markets Frieda mentioned
would offer memorable shopping experiences for your Mom), or the picturesque
villages of Beaujolais, just north of Lyon, recommended in a previous answer
to your query, would be superb fall destinations.
Anne in VA