Minecraft – Self-hosted Minecraft server is accessible for some players, but not others

How are you connecting? Internally or externally (are you getting your ip from the ipconfig command in the command prompt or from a website). If you're not, try connecting internally. Also, I'd really learn how to port forward (there are thousands of tutorials for Minecraft port forwarding everywhere) as Hamachi lets viruses in everywhere and can really mess up an internet connection. Also, keep your firewall on!

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.