M.J.
I would be honest with her and apologize for not sounding supportive initially. Just let her know how much her friendship and family means to you and that no matter what, you'll make an effort to stay friends. I'm sure she has mixed emotions about the move herself.
There are all sorts of ways to maintain friendships long distance. Email, text, Skype and Facebook are just a few tools that can help. And you can always plan visits.
ETA: My best friend moved away when I was in the third grade and it was devastating. Well, guess what ... we're still friends now at 40 years old. It can be done.