( Apologies, this is just search engine fodder. )

If you're someone else who mourns at the demise of "sux" then note that this solution works, for me, on Arch Linux:

In /etc/pam.d/su, to forward xauth keys between users when calling su, add

session  optional  pam_xauth.so

and then a simple "su <username>" from the logged-in user to the user you want to run the X program works.

Full credit to http://askubuntu.com/questions/428284/what-is-a-good-alternative-to-the-sux-command for this.