Frankly I don't understand the problem. If you can file a flight plan from the UK to France, why can't you file the return leg with the same internet application or over the same phone or fax machine with the ATC authority that you filed the original one ???
Most likely becaue he simply handed a paper flight plan to someone at his departure airport, and doesn't know if that will be excepted at his destination for a return flight. If it won't he doesn't know how it will get a copy of that paper plan back to his original departure airport (now his return destination airport) and even if he can, will they accept it from him given that it's not his departure airport for the return leg.
So he is quite rightly trying to find out how others do things rather than waste lots of time on the ground in France trying to figure it out.
dp