There are many solutions for accessing GUI applications as another user (in X Windows). Several are described in this excellent post on the Arch Linux Wiki.
However, I needed a different solution as I frequently “su” to different accounts in the terminal to perform different tasks – I normally don’t just start one application and then exit back to the original user.
I ended up finding an answer in the xhost man pages (go figure). The command below should be run as the user who started the X Windows session. Substitute the actual username of the account for the “su” in place of “USERNAME”.
xhost +"si:localuser:USERNAME"
The si prefix stands for “server interpreted”. This syntax allows us to grant access to a single local user. For more information check out the man pages for xhost and Xsecurity.