Well I am a lawyer (not a French lawyer) and morality and right to privacy aside my legal opinion is that he was pretty naive not to expect this to happen.
And I’d go further and say that the French prosecutor does have some pretty robust provisions to rely on there and probably won’t have to work too hard to get a conviction. People can wail and cry all they want but unless Telegram showed either a willingness to provide chats when asked or else made an effort to restrict access to Telegram in France then there’s not a lot his lawyer can say
This is the main reason I'm convinced that technologies like this need to be maximally decentralized without figureheads that can be used to send a message. Unfortunately, that isn't always a simple task.