And he's also wrong. I assume he's French, and French law specifically says that any unwanted access to restricted parts of a "data processing system" is punishable by a hefty fine and/or prison time. For example, I think bug/vul bounties programs are illegal in France (but IANAL).
Robots.txt is supposed to be public. Its purpose is to recommend not to visit some links (it's only a recommendation). And he did not visit the link itself, he just read the comments (made for humans to read) in a public file, I don't think there is a problem here.
I believe the intent was that if robots.txt claims to disallow a section of a website, then French law would say that section is off-limits to visitors and visiting disallowed sections is punishable by law.
I doubt that this is a valid interpretation of the law. The robots.txt file simply mentions parts of the site that shouldn't be indexed, not which parts of a site shouldn't be viewed by the general public. You use a robots.txt file to prevent a search engine from following links that would enumerate all the possible dynamically-generated content on your site to conserve resources and to prevent junk results appearing for a search user.
Tangentially relevant: when you do have something you want indexed, it should probably be a static page that lives at a permanent URL. But never attempt to use a robots.txt file to "hide" sensitive data.
I believe that only services hosted on the French ground are eligible to the French law.
Also, showing the link but not the content is a smart move because it doesn't prove that the author looked for the content of these documents, when the robots.txt is obviously a file you should be able to consult.
French law would then be different than German law, where the ground the intended audience is on matters. Of course it is much harder to prosecute someone, when the server is in a country that does not cooperate.
A relatively famous French blogger was convicted once in trial for having publicly reported that a governemental agency was letting GoogleBot index confidential/restricted documents : http://bluetouff.com/2013/04/25/la-non-affaire-bluetouff-vs-... [FR] http://arstechnica.com/tech-policy/2014/02/french-journalist... [EN].