What is with people?

[Granyala]

The gross part about this is that the person who lost their cool and insulted the OP is reportable but doing really, really stupid and detrimental things somehow isn't.
In no other situation can someone walk into a group activity, perform terribly, actively disrupt and hinder the progress of their teammates and just walk away with no penalty.
I feel like the OP should be reported for his deliberate poor play. I know there are a ton of gray areas and it's a slippery slope and all that but like this guy played badly on purpose and was actively wasting 7 other players time. That isn't any more okay than saying rude stuff in PF.

First of all: I'm pretty sure that deliberate griefing (displaying his level of deliberate incompetence pretty much falls under that category) of groups is a bannable offense.
It rarely comes to that because groups are way too impatient. They typically kick you after you've caused 2 wipes. That is not enough for a GM to take action on, esp since deliberate griefing isn't always easy to distinguish from real incompetence. It's virtually impossible to do so in small sample sizes.

So, the punishment is the group giving him the boot.
End of story.

Insults are a no-go. No matter how idiotic or how troll-y someone is, you do not insult them.
Stay cool, kick and move on. Kiddos like that are not worth it to get your blood pressure up in arms.
Keeping your wits about you and remaining objective just underlines your own superiority.

For instance: your friend could have written:
"Sorry [NAME], it is not possible to complete this encounter with a tank of your abilities/knowledge. Please read up on your class, the encounter and how to communicate with groups to ensure future success."
It's basically the same message but wrapped in ice cold politeness that, if I were the recipient, would sting 100x more than "You play like shit lol!11".