If he did, it would be a way of unbalancing things, of gaining a sort of unfair power over her -- because the events still would not be things she remembered; they would just be things someone else told her, from their perspective; she'd have no way of judging for herself. I'm not sure what he would gain from telling her (unless just absolution, which wouldn't be fair of him to ask from her, I don't think), or she from knowing.
I'm trying to put into words why I would prefer to be told, if it were me. I think it's partly a matter of principle -- just wanting to know what had happened to me, purely because it's me, my life, my body, etc. I do see your point about how it'd be impossible for her to make a fully objective assessment because she'd be basing it on his version of what happened, but even so, I think I would rather have some information than none.
Now, if I were Neville in the situation, I also think I would tell her, though I wouldn't expect absolution. It'd be more that I'd feel I was holding more power over her by *not* telling, by choosing to withhold that information while she has no choice about it.
Though really, either way he's making a decision for her -- choosing for her whether she's better off not knowing, or not. Which I suppose is exactly why it is a gray area, because that imbalance will always exist.
I do think your way of looking at it is entirely valid; it's just a matter of opinion. I hope you don't mind that I brought it up during the anonymity period when you couldn't respond! It occurred to me afterward that you might take it as a criticism of the story, which was certainly not what I meant it to be.