The FastX Linux Client assumes that the SSH_ASKPASS environment variable is set in your desktop. This is the case for many linux installations (RHEL/CentOs etc). However, a default installation of Ubuntu does not set the SSH_ASKPASS environment variable even if it is installed.
Instead, users must set askpass themselves
sudo apt-get install ssh-askpass
You could then alias fastx2 in his .bashrc to launch that instead