Does transfer at Paris Orly on a layover between Djerba and London City require leaving the international transit zone? I'm interested in both the case where there is checked-in baggage and also the case where there isn't.
The reason I ask is because for nationals of certain non-EU countries the answer determines the need or lack of need for an airport transit visa or short stay visa.
I can not say for sure since you do not provide info about the flights in question, but what I can infer suggests that they do need a visa, even a full Schengen visa and not only a transit visa since you will need to go landside, i.e. pass immigration twice.
The direct flights from Djerba to Orly that I can find are offered by transavia and Tunisair and go to the South Terminal of Orly whereas all direct flights between Orly and London City are offered by Air France or Cityjet of Air France from the West Terminal.
To the best of my knowledge there is no airside transit offered between the two terminals neither in-Schengen nor extra-Schengen. I infer this since there is no such options listed on the airport website nor can it be found googling. This does not mean it does not exist but at this point you should stop relying on the off-chance that it does if you want to spare hassle for your client.
Further source: comment by @Calchas to another answer in this post: "Orly does not facilitate airside transit between the ouest and sud terminals."
At this point it is also irrelevant if there is luggage involved or not.
