If a customer wants access to the backend of their wordpress website but you don't want them to see all the menu options (for example you don't want them to see the theme settings or formidable forms), you can do this by installing to plugins.

Plugin 1 - Webmaster User Role

This plugin creates a new role (admin) in the user configuration that you can then customise with the 2nd plugin.

Plugin 2 - Adminimize

This lets you tick or untick elements within the wordpress backend that a specific user role can have access to. 

Once you have set the appropriate access, it's a good idea to log in to wordpress with the users account to confirm they don't have access to anything you don't want them to see!