The web map needs to be public in order for anyone to view the presentation. The person viewing the presentation could get to the map and make changes, but the wouldn't be able to save those changes to the map, because they don't own it. The could only save a copy of the map as their own. Note, however, if your map references a feature service with editing enabled, and that feature service is not secured, then anyone would be able to edit the actual data. Perhaps your best bet is to make a static map presentation using powerpoint if data security is an issue.
Thanks,
Mike