Check /var/log/Xorg.log. Does the user have the same permissions as all the other users? Belong to the same groups? Is his .xinitrc file any different from the others, and is he using the same xorg.conf file?
If you believe it's something in that user's home directory then:
$ mv /home/user /home/user.orig
$ mkdir /home/user
if that works, then you are correct and need to start the process of elimination.