If the customer has a separate authentication system, then your strategy would be driven by whatever system they are using. There are various libraries that can help, such as django-ldap if they are using an LDAP server for authentication.
You'll need to write separate views that handle logins using the alternate form of authentication, or have a single login page that can use multiple authentication systems if you want everyone to log on through the same page. Both are entirely possible.
Access to pages and resources can be controlled by the built-in Django auth system in most cases, using permissions and groups.
-James