Shared remote SSH session for support

User wants some interactive real time command line help with his Ubuntu computer. He is behind a domestic NAT firewall router (which he can't reconfigure) he has a dynamic IP address assigned by the ISP and he has no idea what it is. He has installed Ubuntu from the desktop CD with all the defaults. His computer is connected to the network but X might not be running. apt-get probably works. wget and curl certainly work. Expert knows what she is doing on the command line and can open a port on her firewall and map it to her computer. She knows her external IP address and probably has a DNS address mapped to it. She has been communicating with user over email/IRC/Twitter/real life and wants to see the situation for herself and co-edit on a console session.