It's a shame he didn't just book airline.
Most effectively AF/KLM through Amsterdam, as there is no reasonable connection at Paris. 1715 from Nantes arrive Ametsrdam 1900. 2120 from Amsterdam arrive Cardiff 2140. Seems little different to the light aircraft times.
Regarding the statement that the passenger had arranged the aircraft himself, I think we would be a bit surprised if he knew, personally, where to start with this. It will be notable to learn who/what else the owner had used this aircraft for over time.