X authentication over SSH is based on magic cookies! The problem though is that when you change users via sudo the new user is not aware of these magic cookies.
Before you issue the su (but after having ssh’ed into the remote
system), request the cookie for the current DISPLAY that’s connecting
to your X server:
bart:~ jordan$ xauth list $DISPLAY
Then, after having done su, tell the new user what the cookie is:
bart:~ root# xauth add hostname.hq.calltitan.com:10 MIT-MAGIC-COOKIE-1
Just cut’n-paste the output of the above ‘xauth list’ onto ‘xauth add’