This question is key because only by acknowledging that there is a problem and accepting help can the road to recovery begin. We would like to have a magic wand that tells us exactly what to say at every moment so that the person opens their eyes and sees the problems and their severity, but often, no matter what we say, we can't get the person to change their attitude. And it is at this point that you despair, feel powerless, or even feel anger, which can lead you to blame yourself or the person or other people close to you for what is happening. These are the attitudes we must avoid; it is very important to listen with understanding and empathy, and not to judge or blame the person with moralizing speeches about what is right and wrong, with phrases like: "Don't you realize the damage you are doing to yourself and to others?", "You are in the prime of your life. With how intelligent you are, don't you see that this is going to end badly for you?" Alone, without friends… Is that what you want?”, “You need to hit rock bottom to realize it, when it’s already too late!”, “You don’t realize how thin/fat you are.”
Approach her with great patience and empathy, and with genuine concern, show her you're there for her. Be persistent, but in a gentle and calm tone. Show her you want to help because you know her and you don't see her doing well, due to mood swings or a feeling that she might be suffering (without making physical references). Be patient, just listen, and wait for the moment when she opens up to you, as this doesn't happen overnight.
The important thing is to seek specialized help and share the problem with a trusted circle of people you can rely on.