Can’t predict if 20 pounds or only 16 would kill your friend, but starving mixed with binging and vomiting is very dangerous. She is already underweight for height. Her healthy weight is 108 to 145 pounds for her height assuming she is at least 18 years old. She has an eating disorder and needs professional therapy from a psychologist/psychiatrist trained in eating disorders.
Keep telling her she looks unhealthy and you are concerned about her health. Offer to go along to an appointment with an eating disorders therapist. If your friend is still a minor (less than 18 years old), you could discuss her unhealthy practices with her family. Better to lose a friend because they get mad at you than lose a friend to an eating disorder.