Great article on how to put storyboarding to work for you. In this particular example, better it would be imo, if the story would lead the app user into voluntarily, i.e. without using the app, ‘caring’ about her friend (this very much reminds me of birthday wishes on FB ;-)
The app might not build a large db of regular users, but instead add real value to people’s life and actions. So, in order to keep your users, maybe the story/plot shouldn’t stop with the ‘letting her friend know how much she cares about him’, but continue with features like keeping track of their friend’s moods, links to related articles on such subjects. An address book with important phone numbers, like hospital, doctor, specialist e.g. A panic button, a diary etc.