At first, it is not clear why the director decided to subvert Break the Cutie from the book. After all, less consequences for the girl mean that the value of the boy's betrayal is diminished. But if one thinks about the film, it becomes clear that Vika suffers as much as in the book (if not bigger, see Fridge Horror below) - she just handles it differently. Thus while the change slightly diminishes the impact of the boy's behaviour, it actually accentuates his betrayal by showing that it is possible to remain oneself and not to crack under extreme pressure. The fact that the boy doesn't muster enough courage to stand up to the bullies is exacerbated by the fact that the girl does.
It takes a rewatch as an adult to realise just how badly the harassment could escalate, were the bullies more determined. And in the movie, Vika probably realised this, meaning that she stood under even bigger emotional stress than her book counterpart.