Don't be too hard on yourself Relly. This is a sick situation with your young friend like an inmate in Wacko or Jonesville, because it sounds as though her mother is heading a cult. People get wrapped up in such certainty and compelling fantasies - like Stockholm syndrome - because the only relief is when 'They' let you feel better and weirdly, you feel grateful in response.
Psychopaths and narcissists brng out the worst in others - frustration, sometimes extraordinary anger that leads to bad actions. That's part of the shame affecting us but not them, because they don't feel it. They preen instead - as in 'see? I said you were a loser/bitch/lunatic'
The story underlines to me why your friend really has to take a sabbatical and get far away. Now she had a record and that's going to really damage her in future. If she takes it to heart, she may behave violently with others later, when in truth she needs a smart therapist who has much expertise in this area of abuse and in mother-daughter ties.
Just keep documentation as thoroughly as a scientist would and be wary of getting obsessed. If you take therapy, make sure the therapist has skill and experience in this area because many don't. Perhaps one question is what this girl and this situation represent for you.
It's totally sensible that your body shows symptoms when your emotions don't. Body tends to speak when emotions and mind don't have the words. Figure out where the aches and pains are and that might give you a lead. And as regards your hope to 'self-improve', no one can improve themselves to cope with, understand or change a psychopath. Take it from me, you can waste years trying (and then more). Go girl!