I'm not sure that you CAN (effectively) do anything. With a psych disorder like anorexia, the person just doesn't listen, doesn't believe what anyone else is saying is true, and won't modify their behavior when it's suggested. They need hospitalization and full time medical care. With this young lady, it sounds like she's not far off a major medical event (eg. collapsing in class or breaking a bone), which might mean the start of her treatment.
Does she have a family member or friend at the gym? We had an overexercising teenager at our gym, and luckily her mum also attended classes and was open to the GFM's concerns.
It sounds like your other concern is for the other participants. I wouldn't worry about copycat behavior. Anyone without this disorder would see that she sticks out like a sore thumb, and not in a good way. If the others are putting pressure on you to 'do something about her' (or you feel this pressure yourself from a control or coaching perspective), you could mention to any who approach you on the side that your hands are tied legally.
Meanwhile, how about during each cooldown, you pull out your 'training tip for the day'? Plan these ahead of time to minimize repetition - consult a PT if you need some suggestions. The training tip for today could be RECOVERY. Give a mini-lecture during the stretches about the importance of recovery between classes for muscle adaptation. Other tips could be nutrition, fluid (something more general so she doesn't feel victimized), the post-workout meal, stretching, time under tension etc etc. That way, the rest of the class knows your intentions are good and you're being as professional as you can under these circumstances.
Good luck! Difficult situation.