Here are some more things to try before giving up:

1. Check that in your friend's server.proporties the server-ip is set to your friend's internal ip NOT localhost or 127.0.0.1
2. Try with a LAN server, have your friend open a world to LAN (with cheats) and type the command /publish, this will give you the port you need to connect to so connect to 192.168.2.16:[whatever port minecraft gives you]
3. disable any and all firewalls, and then try connecting on a dedicated and a LAN server as in suggestion 2
4. Try hosting the server on your end, it may be a mac protection problem or a java problem on his computer. Remember suggestion 1 as you reconfigure your new server. Also try a LAN server form your end.
5. Try a new port/check that nothing is using 25565. Although I doubt this would be the problem its is worth a try.
6. Make sure your router firewall allows your local traffic through, I can't tell you how to do this for your router but give the firewall or DMZ tabs a check to make sure nothing funky is going on.
7. Although I doubt this will work, you can try setting online-mode to false

Best of luck, I hope this helps. I had something like this happen to me one time but with 2 macs and 2 windows, after two weeks it seemed to clear itself up.

Here are some more things to try before giving up:

1. Check that in your friend's server.properties the server-ip is set to your friend's internal ip NOT localhost or 127.0.0.1
2. Try with a LAN server, have your friend open a world to LAN (with cheats), and type the command /publish, this will give you the port you need to connect to so connect to 192.168.2.16:[whatever port Minecraft gives you]
3. disable any and all firewalls, and then try connecting on a dedicated and a LAN server as in suggestion 2
4. Try hosting the server on your end, it may be a mac protection problem or a java problem on his computer. Remember suggestion 1 as you reconfigure your new server. Also try a LAN server from your end.
5. Try a new port/check that nothing is using 25565. Although I doubt this would be the problem its is worth a try.
6. Make sure your router firewall allows your local traffic through, I can't tell you how to do this for your router but give the firewall or DMZ tabs a check to make sure nothing funky is going on.
7. Although I doubt this will work, you can try setting online-mode to false

Best of luck, I hope this helps. I had something like this happen to me one time but with 2 macs and 2 windows, after two weeks it seemed to clear itself up.