Wordpress does this by default, no plugin is needed. In fact that is its essence.
Your client already has a form. You only have to load the content inside the ENTRADAS menu, there you will only load text, photos, video, etc. All similar to a Text editor.
Then the designer must layout the format in which that content is viewed. In other words you have to create a Theme. But Wordpress saves this text content in the database and is usually called POST.
When the developer builds the Theme (set of PHP pages) it dynamically prints what has been loaded from the Entries menu on the Wordpress desktop. (you can access www.dominio.com/wp-admin)
Wordpress also supports creating users with certain permissions. This is to avoid accidentally touching something of the design, etc. then you can give him permissions only to create or edit notes.