Are you sure that you should of gone to the IP address of the server (xxx.xx.xxx.xx/administrator) instead of a FQDN address (http://blabla.com)?
If you were told to insert items in to your host file, this is designed to translate a DNS/FQDN request to an IP Address whilst still retaining the benefits such as passing along the host header (a major factor of most shared and other hosting).
If however you actually typed the IP address as to where to go on your system, this completely bypasses the need for modifying the hosts file, and host headers will not be sent incorrectly.
For example - (I need to spend less time here and actually make a site for myself and company!) but http://www.williamhilsum.com and http://www.ezpcltd.com goes to the same IP address, and resolve to the same site - but host headers are displayed in the brackets on the second line. You can also reach the site via its IP address - http://126.96.36.199 but the server understands all host headers and can do redirecting and host individual sites. For example, for my brother, his site redirects to an entire different site http://www.edwardhilsum.com which is not accessible without the host header.
I hope you understand this.
As for why it worked for the owner, all I can say is he either was using the FQDN and not the IP or was connected via VPN or another method of resolving the domain.
Alternatively, there could be other items such as server IP restrictions, but without knowing more about the service/environment, I can't really suggest anything.
Please let me know how you get on!