Here's a suggestion. Make a file called slooflirpa.sh and put this in it:
Right click on the file, go to properties, permissions, and choose "allow execution as a program" or whatever it says in your file manager.
zenity --error \
--text="Critical Error: 0 Guild Wars PK3 files found!"
Put what ever you want in the quotes. Now, if the Guild Wars Icon is on the desktop, make a copy of it, and move one of the copies somewhere else. Now open it with a text editor and edit this line
except obviously adding the actual path to the file.
If the icon is in the Gnome menu, go to System > Preferences > Main menu, find the icon, right click on it, choose properties, and change the command to the path to the slooflirpa.sh file.
If the icon is on the gnome panel, just right click on it, choose properties, and do as above.
If he's using KDE, I can't help you