If you're connecting to them on port 6000, then no port forwarding is necessary on your part (unless you have some wacky, off-the-wall firewall rule that prevents outgoing traffic on port 6000).
Opening ports only matters when outsiders want to make connections to your computer on a specific port. If you're required to connect to their computer on port 6000, then MisterMe's got the answer -- simply specify the port in the URL.