Fake friend....any tip?

I have a problem with a friend that i met some months ago. the first day we met, she told me with nonchalance her terrible backstory of her childhood, how she was treated poorly by her parents (Muslim fondamentalist), and how she ran away from home (now she lives in a foster home). i was shocked by her story, and how easly she told me all this in the first day. at this point we exchanged ur numbers, and we meet each other at school (we go in the same school), and she is really carismatic and funny, and so i'm happy to be his friend. Until one day she started to insult my best friend at her back, saying she was annoying, sticky, ugly, fat ect. ect. I was stunned, how could she say something like this to my best friend at my face? i was angry, but i decided to let it run for this time (because i was simpathetic with her situation), and to not say anything to my best friend so she wouldn't be sad (and wouldn't beat her). time passes, and summer break start. not founding any work (another story i should tell you), i decide to go to work at my school resturant that opened for a festival as a waiter for all july. i'm really happy about it cause i can work with my friends and classmates, but in the waiters brigade there is.......SHE. since the first day, she did almost nothing: she didn't helped to set tables, she didn't helped to bring glasses, she didn't helped the bar and so on. all she did during the preparation was: doing be reals, doing tik tok's, taking videos, sitting at the cashier chair listening to music, doing live on ig ECT. ECT. Then everytime someone would give her some tasks to do she would come to me insulting them for no reason (of course she keep insulting my best friend and other friends of mine). now preparation aside, our maitre (in this case is our service teacher) gave to me and another person the task of taking orders, my best friend the barista task, and SHE as food and drinks bringer. so at this point you can imagine what is going to happen...anytime she has to take drinks from the bar, she fight with anyone at the bar. then at some point she leave her tasks, and come to the main hall taking orders with paper (while we use palmtops), and commiting errors on errors, and normally the chef from the kitchen would scold me and my other friend for the errors. anytime i tell her the errors she makes she throw tantrums and starts to insult anyone saying they don't work and that she does. yesterday she did it again, she was bringing a jug of white wine to a table that i haven't yet marked it, and a guy from the bar was chasing her to tell her she had to bring the jug to another table. we started fighting in front of clients, and i looked like shit in front of everyone. now how is it possible that she in fact is a spoiled brat? well i ask a teacher and another person about her, and both told me that at the foster home she is well treated (she gets 90€ a month while i get nothing, they clean her room, they clean her clothes, they feed her ect) and that at the foster home she behaves like at work. Now i really want to end our friendship, but when i want to bring the argument, she keeps telling that she is depressed, how she tried to commit suicide, that she hate life and that she want to kill herself. i fell that this friendship is toxic for both, and i can't go on with this moral blackmailing and i think one of this day i will tell her the truth of how she is spoiled and how she expect that the whole world own her something only for what happened to her. 

P.S: the guy that told me how she is treated in the foster home is a army veteran, and told me that he used to torture prisoners till death to make them tell the truth, he studied psicology and he knows all tricks to know if someone is lying, and so he told me that i must end the friendship cause she is full of shit.

P.P.S: he told me his favorite tortures that he made, and how he love to make suffer prisioners killing them slowly (he told that he loved to make prisioners eyes pop with electricity). so he is a bit crazy LOL but for this i'm sure i can trust him when he said that she is full of shit