Run tracecrt to his IP with the VPN on and off and see if there are any interesting differences. Use the option to disable the firewall on your machine temporarily
I have seen people report that they were having issue when they were both on the same ISP. Not sure if that was actually the case of not.
Does ping work via the VPN. Ping many times will not work even when port forwarding is working because you have to configure the router to actually respond to ping....or you must port forward the ping traffic to the server also.
If you can find nothing the next step is the brute force approach. Load wireshark on your machine to start. This will capture all the packets so you should know if your machine is actually sending them. To make it easier you want to use a IP filter so only traffic to that one IP is displayed.
What you may see is your machine sending connect messages to his server but not getting a response. Verify that the IP address and port numbers are correct. You could also have them run wireshark on the server end to see if your traffic even gets there.
Running wireshark with a vpn can be hard, many time you only see the VPN encrypted traffic but I think it now has a option to hook into the virtual vpn interface. But since the vpn works it won't show much interesting.