Well from what I remember of OpenGL coding a PFD is a pixel format descriptor errors stemming from an incorrect PFD usually has to do with bad resolution or color depth settings. As mentioned in the previous post you can try rolling back drivers and also try reinstalling opengl and or change color depth\resolution.
Maybe he can try to change the defalt setting for Tremulous, so the game try to use other config. 32bpp instead of 16bpp, no stencil buffer (?), etc.
Something like this (I have no idea of what exact command sould be)
$ ./tremulous -bpp 32
read the manual will be needed..