How to quit a background user's application

[DominikHoffmann]

Using perhaps AppleScript I would like to have root quit a user's application, even if the user is logged in in the background. What I mean by being logged in in the background is that after logging in another user logs in through the "Fast User Switching" process without logging out the first user. It is the first user's iPhoto I would like to quit from a cron job.

Any suggestions?