Leave her and see where she goes let her find out on her own if she has a problem don't force something you really don't know on a friend
I have only read and watched shows about anorexia. But from what I know, take her to the Dr to get a physical and discuss with the Dr.Then,find a support group as well as clinics that can treat her. Its easier if she wants the help. A neighbor's granddaughter died from this leaving two children behind. Approach her carefuly with love and try to understand, let her trust you and try to work together. You and your family are in my thoughts-Hugs.
idk, if she is in denial then really you can get a support group with her family involved in it....like an intervention. Telling her she needs to go into treatment. Its a mental illness and she will need professional help.