I actually have some degree of empathy for a child under 16-18 who sees violence, grows up in that world, and still chooses not to get caught up in mob mentality- even when they don't come forward to try and change the world.
In some social worlds, the police are the enemy. Calling would be akin to a complete community shunning and maybe even risk of personal safety. To me it's no different than a child not changing the happening within a cult situation. It's all they know, it's the only world they know, and their worldview isn't big enough to formulate the response on how to get out of it on a grand scale, only on an immediate scale.
JMO but IF she chose not to get caught up in the mob mentality, I would figure her a great great opportunity for society to rehabilitate. I would want to provide her resources to education and opportunities. In her world that alone took a lot of courage.
Now if she were laughing and egging them on, that's another thing.