It's probably very hard to find the source and destination of a particular file or request, but it is my understanding that a node knows which file goes through itself even when it is not the source or destination of that file. French law says you're responsible for illegal uses made of your internet connection... So if the bad guys can prove your node sent them a piece of a particular file, it's enough for them.
Well, for now it seems they are only monitoring BitTorrent and (probably) ed2k, and as far as I know no one has been fined/sued for acting as a relay in an illegal file transfer. But if they decide to do so, we may need to go darknet and connect to trusted friends only.
Edit: Yes, I think having as many trusted friends as possible and adding a few limited friends that don't change may be a good temporary solution... Unfortunately, I don't think I'll ever be able to find enough trusted friends to run OneSwarm efficiently with them only (I could only convince one of them to run 1S... It's slightly OT but I think 1S is too complicated to use, and Java doesn't help on the resources usage side.)