Editing a website made with wordpress

WordPress has many roles and you can tell your client to assign to the role according to your work responsibility. If he is ok with FTP then no worries, otherwise you can have one from the below:

There are role according to each work like

• Super Admin – somebody with access to the site network administration
  features and all other features. See the Create a Network article.
• Administrator – somebody who has access to all the administration
  features within a single site.
• Editor – somebody who can publish and manage posts including the
  posts of other users.
• Author – somebody who can publish and manage their own posts.
• Contributor – somebody who can write and manage their own posts but
  cannot publish them.
• Subscriber – somebody who can only manage their profile.

More details can be found here