I like the scarf idea of the house colours. Maybe also a cuddly doll made to look like your friend's favourite character? Personally I would've loved a Dobby the house elf, Neville, or Snape doll when I was reading those final chapters. Hankerchiefs are certainly a must. Perhaps if you're good with sewing you could embroider the Hogwarts crest onto a cloth hankerchief? A blanket in the house colours could also work nicely. By the way, I'm not sure if it's a rumour or not, but I heard that Harry Potter isn't completely over. I read in an American interview that Rowling plans to write some sort of encyclopedia about the Harry Potterverse and the characters. Hope I helped!
I made this a few years ago from an old peanut butter jar and some air drying children's clay. I cut a slit in the side of the jar for the coins to go in and then covered the whole thing with clay. The snout and legs are made around wine corks and the tail is pink wire. Finally I painted the whole thing pink with black detailing and I finished it off by painting the inside black.
As you can see, the piggy's face easily screws off to open but that isn't painfully obvious when it's closed. What do you think?