That's an argument between Cardiff's insurance and the planes / pilots insurance surely? If Cardiff's life insurance find out Cardiff were involved on arranging the flight I guess they will then use that to try and prevent paying.Insurance runs deep in general aviation. It goes beyond the licenses; if you don't have enough hours in corresponding air-frames then insurance won't play ball, because too many successful doctors and lawyers pass their basic licenses, then go out and buy a plane they're not ready for yet and fly it into the side of a mountain.
Regarding Nantes they sold the player, they will get their money from Uefa if necessary, may involve a court case but they will be paid with the PL withholding from Cardiff.