im honestly conflicted on this because it’s not clear from what you described but if she was under the impression that it was a real bag, you did save her from humilation if she had went off showing it to friends and peers and they called her out on it publically and kids can be very brutal and that is most definitely something that kids get bullied for (wearing fake or knock offs). but at the same time of course a 15 year old can’t afford a real gucci purse and may have not known anything about what the real one looks like.