Really awful parents

My neice was being bullied at school by a child who has been kicked out of several schools (according to the kid). 
It escalated at a class party when the girl and her cousin began making physical threats. At that point the other parents in the community intervened and got ahold of her mother. 
I told the mother that the harassment needed to stop and that we would involve police. I also contacted the child who again threatened my neice. I got the impression that she thought this was a normal pattern of behavior, and a normal way for 13 year old children to address adults. 
The screen shot has her DN on it so what she said was "excuse me? She has been bullying my cousin for the last 3 months and I'm sick of it and sick of her! You gotta listen to both sides not just hers! I said I'm gonna fight her and she said when and where and if she keeps going on like this there's gonna be a fight!"
To me, that was a lot of rage coming from a kid. The other kids in their class, and at the party, as well as parents confirmed for me that this little girl came up to my neice unprovoked and started screaming in her face. My neice laughed at her and she left. She then started harassing my neice online, making fun of her and making threats. 
There was no "bullying". Sorry this post is so long. I contacted the mother and sent her screenshots of the one-sided messages being sent to my neice. Her mom's policy when someone claims "bullying" is to take her phone so she doesn't respond, and screenshot all the messages she gets. 
There were over 50 messages from both girls (the first girl and the cousin) calling her a bitch, slut, white, saying she "reeks of cat piss" and threatening violence. 
I assumed the mother didn't know about this behavior but she did. Instead of answering myself or the other parents, she sent a message to my 13 year old neice saying she's a horrible person for "bullying a disabled kid" and "deserves whatever she gets"
After my message telling mom that she might want to consider taking her daughter's phone away for a while, because she had been involving herself in illegal activity, the daughter posted a video of herself smoking a vaporizer and my neice was tagged in it, even though she has blocked the girl. Mom was encouraging her to make this video.
These kids are in 8th grade and they attend a Christian school. I believe that this child escalated the behavior knowing she can't be disciplined by the school in summer. 
What worries me more is that her mom seems to be supporting and even participating in this. Normally I'm a very "their family, their choice" kind of person but this is affecting my neice, her education, and her physical safety. 
She has martial arts training and has refused, multiple times, to allow this girl to goad her into a fight. I hoped that there was another side to the story but there are 30 kids in their class. 28 saw my neice getting bullied. 2 decided my neice was the bully: this girl and her cousin. 
Again though, I don't really care about a verbal altercation between two kids who don't like eachother. I'm more worried about the kind of parent who encourages her child to abuse substances, post naked photos online, go to school wearing see through VS Pink clothes, and does the same herself. 
Honestly, what would you do? Her dad recently lost his job and they have never lived in a bad neighborhood like this. She just called me in tears (the mom lol) frustrated and with no idea what to do. 
They're moving into my house in fall but these kids are harassing her on the internet...