I've had students press the "Guest Login" button rather than the "Continue" button and then do the assignment as a guest without knowing it.
That might have happened to your student. If you know when she thinks she did the assignment, you could check the login.log file to see if a practice user was in use at that time, and then act as that practice user to see if the problems have been completed correctly (the "Show Previous Answers" button could help).
Perhaps the buttons should be renamed; "Continue" seems particularly poor, and should probablly be "Login", while "Guest login" could be "Act as a Guest" or "Do Practice Problems" or something like that. Also, the button could be moved farther away to make it less likely to be pressed. Also, perhaps the background color should change if you are logged in as a practice user to help make it more obvious that you are not logged in as yourself.