Lyon FC and Crystal Palace are owned by the same partnership group. They both qualify for the Europa League but due to the Multi- club ownership rules of UEFA both teams cannot play in the same competition if they are owned by the same group. One has to forfeit the competition for the other to play. Some people argue that Manchester City and Girona were allowed to play in the same UCL competition last season despite City football Group holding 44% in Girona shares and 100% in Manchester City, but the truth is that they placed their Girona shares on blind trust to be managed by an independent UEFA approved panel which secured compliance with UEFA rules and the both teams were allowed to play.
It's the rules set by the authorities of the game, it's not the fault of anyone and in this case you can't blame either of Lyon or Crystal Palace. I don't think that Crystal Palace legal team will achieve anything from this case because they connected to the rules of the game.
They might want to reference Manchester City and Girona case but I believe there is a good reason UEFA allowed that to happen and such reasons should be enough to clear every legal case that might arise.