I'm not sure how you know what she thought or how she interprets the privacy rules. If someone posts something as friends only and someone jacks the link to the image and posts it on twitter, publicly, it doesn't mean the poster doesn't know the privacy rules of Facebook. It means someone who could see the photo took it and made it public. In this case it's the poster who didn't realize it wasn't already public (again, understandably, though in error).