It's not obvious, in my opinion. We know that Henderson arranged the fatal flight, but we don't know who paid him. Or at least, I can't find anywhere that says who paid him. If Cardiff was his customer, forget it. If Nantes was his customer then they organized a dodgy flight as part of a transfer of a guy who they have successfully proven was by then a Cardiff player, and they are very vulnerable.
If on the other hand Solana's agent organized it, because Cardiff were saying "We need him here tomorrow, not the day after" (as he would have been with commercial flights), then I don't see how Nantes can be held liable. Even an argument it's because they messed about and delayed things forcing the rush to get him to the ground would be very difficult to sustain.
French Courts are very direct in their attitude, and 'cut to the chase' is how they do business.