One of my friend's Minecraft gets a "connection timed out" every time he joins my server. I use hamachi to host the server. Any ideas about the issue?
When a connection times out, the server took too long to respond to the client. There are several possible reasons for this:
The network connection is not strong enough to play Minecraft online. I would suggest having 5MBPS download and 1MBPS upload at the least.
Their router could be blocking the connection. Make sure it is set to default settings, and if that does not work try restarting it.
Their firewall may be blocking minecraft from connecting. See this arquade post to allow an exception in the firewall.
Hamachi itself can cause some issues with minecraft, I would suggest using a dedicated host or port forwarding your router instead of Hamachi.