One way to make sure that you can get to that machine on the port is to telnet to it.
Open a command window and type in:
TELNET ipaddress 5900
Where ipaddress is the IP address of his PC.
If you get a response like "RFB 003.007" then you are connecting to the VNC server on his PC (User ctrl-C enter to exit). The problem is then the password.
If you get a response like "Connecting To ipaddress...Could not open connection to the host, on port 5900: Connect failed" then you are not getting through the router or the firewall.