L.L.
S.H. has some great suggestions. Try to emphasize the ways they can stay in touch and don't forget that to a 6 y. o., emailing (or even snail mail) can be an exciting, grownup kind of thing.
I wouldn't minimize the importance of what is happening, but I'd be careful about canceling trips or constantly worrying about it. It's sad, but friends moving away is one of those things that will come up in life again, and again. My 9 y.o. has also experienced it, and yes, it was a wrench. In her case, email filled the gap for a while and they had a great, fun correspondence. Recently it seems the friendship might be petering out a bit, which also happens. Maybe it's best that we help our kids see that these episodes are an opportunity to learn how to keep friendships strong, but also live through their transitions, too.