Alright, I may not be any help, but I can relate exactly. I have literally been in this exact situation.
I'll tell you my outcome, since I have no real advice.
I had a best friend for 7 years, me and him knew everything about each other and we hung out every weekend. I met a girl, and I always treated her like my sister for 3 years. Eventually, she got ties with every single one of my friends, including my best homie. I won't type the entire situation, since, literally everything that you're going through scarily is exactly how mine went.
I was faced with your decision, drop her or stay and try to fix it. I stayed and tried to fix it.
Everything went wrong. Because of the extent of her poisonous attitude, I lost my best friend, and he never wants to see me again. All of the friends I've had have deserted me because she spread rumors about me when I tried to fix her. It was too late to go back. Everyone already blocked me after she did her damage.
I personally would break it off, but I have no authority over what you do. I can only say, I wish I didn't stay and try to fix it, maybe things wouldn't have gone so intensely wrong for me. I really hope you don't feel the same regret that I do.