I've never heard of a 'standard' contract which in some way prevented an ex-employee saying anything about a company unless it involved the disclosure of confidential information.

probably like in many countries a contract of employment isn't obligatory in France though most employers make out a contract - almost every contract i've signed in France has had a confidentiality clause included as well a 'non concurance ' clause ie to say that you won't set up in business within say, a 100 radius of the ex company should you leave the company voluntarily.
The standard confidentiality clause says that yo won't disclose company information to a third party which may be to the detriment of your company.
In my case,the ex employee (and ex-friend) found alternative employment (better job,higher pay etc) but was bound by the terms of his contract to serve three months notice.The man asked if he could leave earlier than the three months in order to start the new job early (for the higher salary etc) and the company agreed so long as in return,he would sign a document stating that he would not testify against the company in my favour as he had originally agreed to do.
The agreement is between him and the company and is not illegal however morally wrong it may appear to be.