First, make sure that other users can log in successfully. That message could indicate that Alfresco is down.
Second, site membership does not determine whether or not someone can authenticate. What matters is whether or not they are listed as a user. So, go to the admin console and search for Colleague001 in the user list to make sure the user is there and that everything appears okay with the user entry.
Another problem could be that Colleague001 is in a different part of the LDAP directory than users who can authenticate successfully. It might be helpful to look at the user in LDAP and compare the entry with a user who can log in successfully. If there is a difference it could be a good clue. An example of a common problem is when the user is moved to a different part of the directory tree and the LDAP configuration in Alfresco does not include that part of the directory in its search base.
Last, if everyone else can log in successfully and the user is okay in Alfresco and in LDAP, it could be that Colleague001 is using the wrong password. If you have another system that is using the same LDAP directory, have them log in to that system using the exact same password they are trying to use with Alfresco.
