I'm reading the edited version:
You say your daughter has made a real friend... but that she's a military brat, which has you nervous that she's just going to be leaving in 2 years.
2 things:
1) 2 years is a lifetime to young children, and a VERY long time to older kids/teens. If she has a good friend in this girl, encourage that. In 2 years time the situation may have changed entirely, because if she and this girl become besties your DD may have changed / blossomed under having such a great friend and be more than able to make good friends by the time this girl leaves.
2) I was a military brat. I can not only list off the names of all of my besties (Bernie in preK and K, Stephanie in 1st & 2nd, Aly in 3rd & 4th, Mike in 5th & 6th, Bonnie in 7th & 8th, AJ in 9-12) BUT I am also still in contact with ALL of them except for Bernie. (I was too little, and so was she... neither of us were writing or really able to phone). They were all wonderful, deep, and caring friendships that we maintained as I moved, and whom we have visited, phoned, and written to. And we didn't even have Skype!!! (free video conferencing). Something I never had to experience, either, was "outgrowing" friends. Because I was gone, I was a "safe" friend. We didn't fight being 3-5k miles apart, and so we all maintained those friendships relatively easily through troubled hormonal years. And now as adults, we still stay in contact and visit.
So I would say, not to limit the friendship, just because this girl will be heading off to parts unknown in 2 years. The added bene, of course, is that ANY family could move away at any time (job changes, divorce, death in the fam), but with military families you KNOW they are leaving long in advance, so you can prep your daughter for it... like by trying out skype).