Now this is the text, and it speaks explicitly to the point. He gave her something that could kill her because he was hungry. If there is another way to read that I'm open to persuasion, but barring that, it is what it is. This is some of Jim's weakest work. It makes Harry look stupid. This is a college student perspective, which is what he was when he wrote this. If it was as dangerous as he states it, then he had no business giving it to her. If he doesn't then she can't hurt herself with it and it doesn't matter if she listens or not.
Maybe Harry had no business giving it to her, but still, unlike Korean kid, Kim had a teacher available and chose not to listen.
I am perfectly willing to continue this other discussion if you acknowledge that, okay, Kim's situation isn't like Korean kid's, because for my original argument this isn't a relevant point.
Now then, on to "should Harry have known better" - yes, he should have, and does; Fool Moon's first page is Harry explicitly spelling out he knows better. Only after the twin pushes of Kim saying it is academic only (a lie) and the food (a bribe) does he do it, which is evidently sub-optimal, but it is when the
lie is proven wrong that he backtracks again (a correlation to his having food is there too, I guess).
So Harry's first instinct was 'give no information'; probably safe, but got lied past by Kim's choice.
His second approach was 'give limited information' which failed.
His third possible approach was 'give all information' which Harry thinks would inevitably fail too, because even with that information Kim couldn't do it (and since she was lying about the why, Harry couldn't do it for her - even if he had the ability, which I am not sure of).