Because best case scenario, they'll make you violently ill. Along with anyone else unlucky enough to eat it.

[She was hoping she didn't have to give him the worst case scenario.

She briefly ponders just leaving it at that, letting him make his own way from here on, and then decides against it. After all, those aren't the only poisonous thing he could find around here.]

Come on.

[And then she heads off to find some mushrooms he actually can eat.]