From my knowledge, if you and your friend on on the same LAN together, neither of you will be able to view the computer in the "Internet Games" section if the server is also on the same LAN as you and your friend. However, your server will be viewable to others OUTSIDE your LAN.
For example, I run the ~].SL.[~ Server on my LAN, and neither my roommate or myself can see it listed in the Internet Games section. However, we both type "slist" into our console and it will display the servers on our LAN. We then type "connect x" where x is the number that is listed for our server. It will directly connect us to the server without problem. For others not on our LAN, they will see it in the I.G. list. This is proven because my server always has people in it and I don't advertise its IP nor have ever shared it with someone.
Outside the router, people WILL BE ABLE to view your server, so no worries. If you think its a matter of port selection, then why not try opening up your ports from 27000-27500? Even though ESF doesn't use ports that high (unless you have specified thru hlds), its not too much of a security threat to open that high. Also specify in your router config WHAT LAN IP your SERVER MACHINE is connected from. If your router has assigned the server machine with address 192.168.0.3, then in your router config, you need to specify this with the opened ports. Make sure to use both TCP and UDP if your router has those options. On my netgear, It automatically lists for both.