A friend's son was like this. He just refused to eat when he wasn't hungry.
It turned out his digestion was a bit sluggish (no problems as such, he just took a while to process food) so he just felt full for longer, somewhat perpetuated by him drinking a lot.
Her GP, after many visits and weigh ins, sent him to a dietician, who just said he's happy, he's not ill and he does eat so why are you stressing?
He also suggested that her son was getting rather a lot of attention over it and pointed out that when she was worried about him not having eaten enough she had a bit of a habit of buying his favourite foods or taking him out for a 'treat'.
Basically, there was no motivation for him to change the behaviour because he was having a great time!
I'm not saying you do that, OP, but something to think about.
If I were you I think I'd try to chill out a bit, back off and eat well in front of her so that she is seeing healthy eating patterns and just see if it changes over time,
She might just be really in tune with what she needs and not really driven by what she wants , which is great.