If they use XBMC you can ftp in (and with Flashfxp) press ctrl+r
Enter
QUOTE
site shutdown
in the box that appears and press enter.
The xbox will now power down :-)
Wont work when playing games and only with XBMC afaik.
Btw, here´s the rest of the raw commands you can use with XBMC
R] 200-FTP SITE HELP
[R] Help This help message
[R] Reboot Reboot the xbox (power cycle)
[R] Restart Restart the xbox (power cycle)
[R] ShutDown Shutdown the xbox
[R] Dashboard Run your dashboard
[R] RestartApp Restart XBMC
[R] Credits Run XBMCs Credits
[R] Reset Reset the xbox (warm reboot)
[R] ActivateWindow Activate the specified window
[R] ReplaceWindow Replaces the current window with the new one
[R] TakeScreenshot Takes a Screenshot
[R] RunScript Run the specified script
[R] RunXBE Run the specified executeable
[R] Extract Extracts the specified archive
[R] PlayMedia Play the specified media file (or playlist)
[R] SlideShow Run a slideshow from the specified directory
[R] RecursiveSlideShow Run a slideshow from the specified directory, including all subdirs
[R] ReloadSkin Reload XBMC's skin
[R] PlayerControl Control the music or video player
[R] EjectTray Close or open the DVD tray
[R] AlarmClock Prompt for a length of time and start an alarm clock
[R] CancelAlarm Cancels an alarm
[R] Action Executes an action for the active window (same as in keymap)
[R] Notification Shows a notification on screen, specify header, then message.
[R] PlayDVD Plays the inserted CD or DVD media from the DVD-ROM Drive!
[R] Skin.ToggleSetting Toggles a skin setting on or off
[R] Skin.SetString Prompts and sets skin string
[R] Skin.SetPath Prompts and sets a skin path
[R] Skin.SetImage Prompts and sets a skin image
[R] Skin.ResetSettings Resets all skin settings
[R] Mute Mute the player
[R] SetVolume Set the current volume
Remeber: site [command]