it's attention seeking behavior and is best ignored. Anyone with a genuine desire to starve without this being noticed would not be commenting on how they haven't eaten. Smile faintly and say "Oh. Ok." And then change the subject.
As you like her, it's worth doing this, because the other alternative is to stop talking to her!
I hacve a long time friend who, as a teen (and 2 stone lighter than me), used to whine about her tiny size 6 frame being "Fat" and "Bloated" and would ask constantly "Do I look fat, God, I've put on so much weight, do I look fat?"
and at first, the flurry of reassurances that no, she was not fat, seem to sate her need for attention ... but it continued. And one evening, she had upset me with it and I was moaning to my mum about it, who gave me some advice ...
So the next day, She started up with "God, I am so fat, I must be a stone overweight, " etc etc, and I mentioned that Weight Watchers had free registration that week. She was stunned. She said, "Do you really think I'm fat?" and I told her that it didn't matter what I or anyone else thought, if she felt fat, she should go to weight watchers.
And, in 14 years, she has never ever mentioned it again - except this year, occasionally, because we are now both overweight!.