I think it'd work best if she does her voice over, like in the first movie, and mentions that she has been hearing his voice in her head, even though she knows it's not really him.
How they'll work that out, I don't know, but it will be hard for non-readers to get.
In a way, though, I'd prefer if they just made the movie as is, without trying to explain it to non-readers.
That way, they can spend more time on the important things.
30 year old tattooed metal-loving male Twilight Fan. Breaking the Stereotype.
If you don't understand the science of Twilight, or just like to scoff at it, read my explanations here