They will need to click on File and select Customize Scoreboard, and then click on the 'Reset Tree' button to reset their scoreboard to the new default you have created. But be aware that they will lose any customizations they have done previously, such as adding other stored queries to their scoreboard, or if they have saved any searches in the 'Saved Searches' folder.
Unfortunately, there isn't a way to reset their scoreboard to the default and save their previous customizations ('merge' their current scoreboard with the new default). If there isn't an idea posted to add this functionality to the customize scoreboard screen, there really should be!
Tammy