I agree with the above advice. I've been anorexic almost my whole life. I am very used to people inquiring. And I am also pretty stand offish about it. If someone starts to seem too curious, I just back away from them. It's really hard to help someone with this when you don't know them well. I'm not trying to discourage you from trying to help, just be careful about your approach.

If someone starts talking to me about what they ate, I'll listen but I won't respond back in turn. If someone directly asks me what I ate, I will lie. Because I have been questioned like that for so long, I learnt to lie about it or be vague to keep anyone from continuing to ask.

Good luck. I think just talking about food casually would give you some insight. What to do after you think you might know? That I don't know.