[ he does try to stop her, reacting instinctively to the news that she had cared, as a friend, as (perhaps) more then that. It takes only a moment before he's moving, not to touch her, but to move in front of her, to block the door that would lead her into the light and away from him once more.
He never knew how strongly he wanted her to stay before he had lost her. ]
Ah... wait! Please. I... [ he may barely have time to get the words out, but, what he wants, what he wishes to say is simply, "I want your happiness too." But that's a selfish wish. He's lost all rights to that.