As has been mentioned/alluded to above, it depends on the nationality of the traveller (with other exemptions also in play, such as having residency permits/green cards etc for the destination country, depending on which country is the final destination).

The Indians you mentioned above would not have had a Schengen visa (so therefore couldn't enter the country, and therefore had to remain in the transit zone in the airport) but they almost certainly required an Airport Transfer Visa.

The final answer is: always check with the nearest French embassy or consulate.