One of my friends just recently died and I had a hard time getting through it and I still am. My other friend came over to my house who wasn't very close to my dead friend, and said that even though she was one of your closet friend she will still be with you and still be close, there are 7 billion people in the world and god put you with her and had you two be friend, she will still be with you.
That it always gets better! Everything happens for a reason and good things come to an end so better things can fall into place! There's over 6 billion people in this world, she will have plenty more best friends out there and she will find and meet a lot of new people
Assure her she isn't alone that you are there for her, do something sweet randomly to put a smile on her face (like make her cookies or write her a nice note.) remind her just cause her friend is moving away she doesnt have to move out of her life, she can still txt, call, email, write letters, Skype/Oovoo, and theres always Facebook. Just be there for her and listen to her.