That's a tough one.....not easy to approach someone you don't really know about something as sensitive & personal as anorexia & expect them to be open with you about it. Usually people are in denial in the first place.....not sure you would really get anywhere with the person anyway.

You might just start to taking to her & in conversation bring up something about what you eat for breakfast, lunch & dinner & see what she says about her own eating habits so you can see if she is actually anorexic or just has a metabolism that is so high no food goes to weight. There are more reasons for being thin than just having anorexia.....but it is the most obvious thought for being thin.

Good luck with this.....I know you are just trying to be helpful.....not sure just how well that desire will work in reality.