SSH window forwarding with Xming

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http://unix.stackexchange.com – I have installed fedora 17 on the old desktop computer and put it to DMZ of my NAT network - i.e. whole computer is accessible via public ip address to the internet and could be called "server". I've allowed SSH connections on firewall - everything works perfect, but there is one huge problem. Both me and my friend abroad have Windows 7, so we are using putty + xming to forward windows. My computer (on the same LAN as "server") works without any problem (setting putty like this http://www.geo.mtu.edu/geoschem/docs/images/putty_4.jpg). Friend, however, he can't open any window (xming is run (HowTos)