A durable picture frame with a picture of the two of them, perhaps a bracelet, ring, earrings, or neclace so she has something to wear that will remind her of her friend. I would say a nice little starionary set but I don't know how "into" writting letters kids these days are. I just think it would be cute for them to have a matching set so they can write each other letters and having the matching set would make it special.
Maybe your daughter can write her a little letter to stick in the basket telling her how much she means to her and how she'll miss her or something like that.
I am sure anything you come up with will be special to her. Hope all goes well!