Its not a libel case actually, its a criminal proceeding that could (or could not) lead to prosecution for cyberbuling, which is a type of penal infraction in France. But from what I know of American law (granted, I know more about other systems), the USA tends to also protect its citizens from other types of judgments as well, and like the majority of western countries, it doesn’t extradite its nationals. I believe the UK is the same way.

You are correct in regards to British Libel laws, but I don’t actually believe this is all that relevant in this case since… its not a libel case, and it wasn’t filed in the UK.