A.V.
I have been the kid that moved. I would do a few things. I would remind her that June of 2015 is far away. She can have many adventures with her friend in the meantime. They have a whole school year! And even if there is no bon voyage party for the family, I would arrange a special event or sleepover or something so the girls can have a hurrah. Where are they moving to? Would it help your DD to learn about where her friend will be, to be reminded about Skype and texting and emails and letters. Who doesn't like mail? Something my friend did recently was have a going away party and had the kids give out their new info in the goodie bags and receive info on scrapbooking pages done by each friend. Understand her fears and concerns, and try to get her to look at the positives vs worrying away the next year. I also agree not to dwell on the move. It's a thing, but it should not be at the forefront of every day.
When my DD was worried about her friend moving (though not her best friend), I said, "Do you remember my friend Ms. T? And how she moved to Iowa? Do you think we stopped being friends because she moved? Not at all! You can have friends all over the place." And then we talked about friends and family all across the world. It seemed to make her feel better.