I think failing to some extent is fine, if the feedback is good and the device returns to a clean state. Even on a phone with buttons, she sometimes fat-fingers the wrong button super close to the phone-book-down key and ends up in the main menu, scrolls down 10x, then tries to hit the dial button for e.g. display settings. Phone appears broken because it just doesn't dial, and neither resets itself for invalid input so she could start over. She has no mental map of the various other states the phone might be in, and in confusion forgets that the hang-up button also returns to the home screen. Things are less than ideal.