Keypad Library... Returning a string after user pressed a certain character

Projects Programming
6 
  buffer[i] = customKeypad.getKey();

What did you store here if no key was pressed? Why do you want to store that?

  if (buffer[i] == '\0')

This is a rather poor test. The getKey() function returns a #defined value, NO_KEY, when no key is pressed. Assuming that NO_KEY is equivalent to NULL is a poor assumption.