Some people do cause because it's trendy or cool. In many cases, someone who is cutting because they are actually feeling emotional pain and that's what drives them to hurt themselves, then in most cases they might even be ashamed of the cuts and scars and try to hide it rather than show it off.

The thing is, is your friend actually cutting because she is upset, or because it's trendy? You should talk to her to figure it out, because either way it could develop into a harmful pattern when at first it was just a pissing contest.

Also, in my opinion, if the other girls are cackling because the girl didn't really cut, then you should both take it as a good thing. Cutting isn't trendy, it's harmful and unhealthy.