"You must miss him an awful lot," she says, and right away he knows she knows. She's fifteen years old and the only one who's said it. Either everyone else knows, and just don't know what to say to him, or they don't have the faintest idea, and assume that he just feels the way the rest of them do, having lost a friend and nothing more.
"Yes," he says, because that's all there is to say.
She's too young for him but he suddenly doesn't care. He knows he's been wandering around all summer, almost like a ghost (but not quite, because if he were then there wouldn't be this problem, would there, they'd both be dead and he wouldn't be left here all alone) and he is suddenly sick of not saying anything, of not being able to tell anyone how he feels, of not even being able to talk to Harry about it because Molly thinks he's too young to know, and so he kisses her.
She's too young and it's all wrong but she's the only one who understands, right now, or at least who's willing to approach him, so he will take what he can get. A distraction, a cure, a solution, some way of dealing with the colossal gap that's been left in his life.
And when he cries into her hair, afterwards, she tells him that it's all right. It's not, none of this is, but she knows he needs this lie.