It is wonderful that you want to help her, because it will be a long and not always happy journey.

When she yells/is cold at you, it is her illness talking, not her. She is still that warm loving girl you knew, but is in a lot of pain.
Friends/families may love and do their best to help them but this is a serious illness and can't be handled by untrained loved ones. Trying to force feed her and reassure her that she is not fat will just cause you to become more upset and frustrated, and victims will become more secretive and lying to avoid food. The most helpful thing you could do is get her into some type of treatment and be there to support her through it. It is not your job to fix her but you can really help by being there.
People say that the earlier you seek treatment, the better the outcome. Is she seeing anyone? could you possibly take her to the doctor, because there is probably a small part of her that wants help but you can't provide it.

And also just as importantly is to look after yourself. What use will you be if you get worn out?

Good Luck, you are doing really well asking for advice