I agree with Lea here. I honestly don't think Lily would slap him and although her words might make him act a bit more responsibly for a day or two - that would be all.
For him to be 'dead inside' for eight months or so, merely because she told him off in front of the school would be OOC. If you need a reason for him to be miserable then it would have to be something cataclysmic like the death of someone close to him. Lily's scorn, to be honest, would be like water off a duck's back.