More than likely he's using Xorg's dri and default 'nv' driver.
From a terminal, type
sudo apt-get install nvidia-glx-legacy
then,
sudo gedit /etc/X11/xorg.conf
and scroll down to the "Device" section and change the line which says 'nv' to 'nvidia'.
If that don't work, type the following to launch tremulous
tremulous &> start.txt
and post back start.txt using QUOTES from the forum interface please - don't just dump it all without using QUOTES.