A girl in my younger brothers class got expelled from school for things she wrote on Facebook.
She made a post about her tutor group and how she hates everyone in it.
Making sure to insult most of them personally, including swears and curses.
She then also went on to insult their tutor, going into excessive detail about her being a slut (The woman is a married 50 year old Textiles teacher) and other obvious lies.
Anyway, my brother and his friends, since she was stupid enough to comment about people directly and openly, they saw her post, printed it and took it directly to their tutor.
Which got it sent to the Year Head and then furthermore onto the Headmistress of the School, and she was then expelled.
However, considering what you've posted about messaging, I wouldn't worry about it.
Your friend isn't likely to send it in, and if he does, either make it clear that he is just as bad, or take the punishment, which would definitely be little more than a warning or a simple detention.
Don't go fretting over it. It's not a big worry. Just be careful not to insult people over the internet who you know.
Just because it's online doesn't mean you can't get in trouble for it.