The solution is to first close the keyboard viewer, then use Terminal (under Applications/Utilities) and enter the following command at the prompt:
defaults delete com.apple.KeyboardViewerThe keyboard viewer should be restored to its original state next time you open it.
Another issue my mom was having was that when she tried to enter Chinese, certain characters cause programs to crash (also known as 地雷字, di lei zi, meaning “landmine characters”). It turns out the culprit is the "Use symbol and text substitution" option. It can be turned off under Preferences, Language & Text, Text. Text substitution erroneously matches some input sequences prematurely from the input method and causes invalid characters to be entered into the program.