I would contact his friends and tell them the truth about her. Tell them you are worried for him and he needs support to see that this is bat shit crazy behaviour.
Clearly the whole kill your Mum thing is such a teenage thing to say. What happens next? Your son gets arrested for murder and she never sees him again. Did she think that he would just be okay to kill you and then they waltz off into the sunset? This is where teens don't think things through. I would want to point that out to him so he can possibly point that out to her if it ever comes up again. And yes, teens in the US have done this, killed a parent to be together. It does happen.
I do wonder that if the roles were reversed and the male was the one saying kill your Mother, the police would be taking a much harsher stand on this.
I would also have printed out the stuff she wrote about you to give to the parents to confront her attitude in black and white. I do think you are doing an amazing job in all of this.
To those saying ban them from seeing each other, someone I work with did this with her 17 year old daughter and she just moved in with the unemployed, drug taking, never had a job 24 year old who still lived with his Mum, also on benefits, no rules household. It was devastating.