I think the install should have asked for a root password. If he just hit "enter" and did not assign one, you can set a password for it.
Open a console and type
su (enter) "if it asks for a password just hit enter again"
whoami (enter) "to make sure you are (root). If you are not root at this point, Don't go any further."
passwd (enter) "follow the prompts to assign a password"
Again, it's just my guess that a root password has not been assigned.