**Network Error - Unable to reach the server
If you're not having this trouble but your colleague is, a very common issue is sessions on www/no-www or HTTP/HTTPS URLs.  CiviCRM's base URL is hard-coded in the settings file, and while you can follow relative links around the site for a while, some links will be generated from that URL.

Let's say you have a site that's accessible at http://www.example.org and http://example.org.  civicrm.settings.php has http://example.org as the base URL.  If your colleague logs into Drupal at http://www.example.org/user and follows links from there, everything may be fine until he hits a URL that CiviCRM generates from the base URL.  It'll be http://example.org/whatever, and Drupal will deny access, since there's no session at example.org--just at www.example.org.

I don't think it's necessarily precisely this, but I think it may be something similar.  Maybe your colleague logs in at http://www.example.org instead of https://www.example.org.  Maybe you have SSL enabled only for www.example.org, or maybe there's a redirect that's cached funny.

The way to avoid this is to make sure three of the following four URL variations all redirect to the fourth:
 - http://www.example.org
 - http://example.org
 - https://www.example.org
 - https://example.org

It doesn't matter whether you pick https://www.example.org or https://example.org as your official URL, but have everything redirect there and set that as the base URL in civicrm.settings.php.

Although I think something like this is probably the issue, Jon's answer is a good general process for debugging this sort of thing.