Did she know what he'd remember? Hows that? I'm sure he had that hat for more than just the night the girl died, both before and after, and she was just shooting for Nathan memories. Since the Haitian erased his memories and she, as far as I can recall didn't know Sylar had the memory trick (or that Nathan was accessing Sylars powers at all) she had no reason to think he'd remember that piece of Nathans life. She was just hoping to strenghten his Nathans memories by giving him his shit, like when you come across something from when you were a kid and go "Oh, wow, that was so cool".
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