See my comment downthread--- I think that in this case there is likely to be a causal relationship between the approach that LW takes to their child's life overall, including but not limited to their electronic communications, and the child's attitude that anyone who doesn't live, including in their use of electronic communications, the way that child and child's friends do is being not just weird but actively rude to them--- i.e., not just different, but being different at them as a deliberate act of... contempt? Dislike? Insult? IDK the exact emotional valence but I'm seeing a connection between "LW is policing their offspring's communications with peers for what LW thinks is appropriate" and "offspring thinks that anyone who does not communicate with peers in exactly the way that offspring and friends do is being deliberately insulting to them". I think some other commenters covered that from different angles as well.

I don't know what to tell you in specific about what's appropriate for monitoring your specific kid's communications with peers, including the electronic kind. I am going to say that this stuff sounds like it's not dissimilar to the hell I got from age-peers to my face throughout elementary and high school, it's just that now the adult authority figures have documentation of how awful kids can be to each other. I also really, really like conuly's suggestion that you tell your kid upfront how much monitoring they can expect of their electronic communications before they have that capability.