THANK YOU for raising this point, because I've been trying to frame a comment here to the effect that I think there is a very strong causal relationship between "parent thinks they should have unfettered access to all spaces of child's life, including but not limited to electronic communications" and "child thinks that anyone who does not behave exactly the way they do, including in their use of electronic communications, is being rude and awful and deserving of social abuse".

Also, I bet that Millie does indeed have social media accounts that are carefully firewalled from her awful meatspace peers as IMO the internet was intended to be, namely an escapespace for people who don't conform to the dominant ingroup, not a place for the dominant ingroup to police them still further and into oblivion. /is an Old