How to configure remote assistence over a NaT firewall?

Hey i am having some trouble trying to configure remote assistance for another computer. I want to try and provide technical assistance for this user because he is absolutely a green horn when it comes to computers, so it will be very beneficial if i can just remote connect to his pc when ever i choose to and he can give me access using "remote assistance" I have already tried doing this at my current location. I had both the host PC and my PC here at this location, i used the remote assistance to gain access to the computer. All i had to do was configure the windows to allow remote assistance which was not that hard to do. But now that he has taken his computer back home, he is now at a different location. When i try to use my computer from this location to connect remotely to his location. The remote assistance fails and i can't connect or gain access to his desktop. I do happen to know that he has an extra wireless model or firewall that he is using for his network and my guess that some how its not configured properly to allow another HOST to connect remotely over a NAT firewall, Reason that it worked here is because i was not remotely connected over a NAT firewall, i was connected from a private network to a private network. Which is my same network. Now i need to know how connect to his public network using "remote assistance" and how to configure his computer properly to allow access. I know bout portforwaring IP address 3389 because thats the IP that allows for remote connection, but doesnt that only work with "remote desktop connection" or is it necessary to port forward that through their router for "windows Remote assistance" as well. After trouble shooting the problem, its clear that the problem does is not from my remote location, but it states the HOST user is not properly configured to allow remote assistance. Any assistence or help in this matter?

Jeff, give CrossLoop or TeamViewer a try. They both do the required setup at each computer so it's more likely to work.

Ok so basically with "crossloop" i install the application, then click on the share tab. Give them my access code displayed thier. Then the HOST machine installs the application. They click on "access" tab and type in the code that i gave them and then they click on "connect' then i can gain access to thier machine remotely? Or do i have that backwards

Yes, that is exactly how it works. I use it to help my family and friends. Works very well for me.