Does he get this error only for your server, or for others LFS servers as well? He might be having an issue with his internet connection.
Is he using a wireless internet connection? This is sometimes known to be an issue - use a cable (however, I've been using wireless to race for 2 years, no issues for me!)
Can anyone other than your friend connect to your server? This connection error could be due to your host/server not having enough bandwith. Get someone else to join in see if your host works okay for them.
He can do a test at pingtest.net to check how much packet loss he's getting. If its much higher than 0% it would result in a "loss connection".
Open the Windows "Comand Prompt" (cmd). In there, type
"ping master.liveforspeed.net -t"
It would start to ping the master server. Leave it running and get your friend to open LFS and connect to your host. When he get "lost connection" switch back to command prompt and press "CTRL+C" to stop the pinging.
You can see the result:
i.e. Packets: Sent = 155, Recieved = 155, Lost = 0 (0% Loss).
You can also give him your IP address (not adviced!) to ping. You can find out your IP address (open up pingtest.net and you can see it) and give your IP for your friend to ping (ie. "ping 213.40.20.2 -t") before he's connecting to your server. He can check the packet loss after he gets "loss connection" from your host by pressing CTRL+C.
Edit: Just tried to connect to your(?) "[ASS] Any Style Server", but couldnt connect as I was getting client connect error (associated with no port forwarding). Just for you info.