My wife has a friend that used to live here (Colorado), but now lives quite a ways away in Oregon... and they talk on the phone constantly... constantly!
She thought it was going to be horrible, but they are still very much in touch, and they have a great time together when they visit.
My point?
It sometimes hurts when a friend moves away, but it hurts much more to lose a friend...
So go to the party. Tell her you're sad she's leaving... do your best to smile, have a blast, and send her off laughing.
Stay in touch.
Visit when you can.
keep smiling,
Ben