fpar - when you delete an application that has a shortcut on the Dock, the icon gets replaced with a "?".
Tried restarting his Mac?
All else failing, locate the file "com.apple.dock.plist" from /Users/<username>/Library/Preferences and drag it to the desktop. Reboot and the dock should be restored to default settings. If so, delete the plist file from your desktop and have your friend rebuild it the way he/she likes it.
Thanks that worked, but now Finder's icon is not displaying, same as trash. Ill try a little bit here. Any ideas how to get those 2 icons back?
No, he is not using any app whatsoever.No idea. Is your friend using any 3rd party apps that modify the Dock?
Remember not to delete that plist file from your desktop until the issue is solved. OS X will always create a new one if it can't find one, but it's good to have the original in case you need it for anything.
Create a new user and log into the new account.