Was that really it? She was that upset because she thought Seungeun had given up and didn't care about drawing as much as her? I would certainly understand her being disappointed and frustrated since they were friends aiming for the same goal, but it doesn't fit this big of a blow out. We know that Seungeun was misunderstood, but even if what they thought was true how does that justify not only ending your friendship but ostracizing and bullying her?
Also I know they're teenagers and might not think everything through but it doesn't take a genius to think of asking her why she quit. Did any of them try to see if she needed help or support? If they went this far while knowing so little and making hardly any effort, doesn't seem like much of a friendship to begin with. That said it seems like the red head may have blown up the issue bigger in her jealousy, so we'll see.