To a friend, who’s going international and not returning, to clarify.
And the short answer is, you can’t.
If you know it’s the last time, you’ve just got to look at them and try and make yourself remember this moment a little better than it remembers everything else, but it can’t really. You have to live in the moment a little better than usual, but yeah, you can’t.
You can’t say “goodbye” in such a way that wraps up a life time. There’s no thesaurus with the word to sum up goodbye well enough for a forever.
Can you trust your friend enough to know that your “goodbye” means good luck, be safe, be smart, have a nice adulthood and a nice retirement? You’ll just have to. You both have to pretend that you don’t already know you’ll fall out of touch, and that you won’t visit and neither will they.
“Goodbye” is the best you can do. Pretend it’s not the end, because the word can’t live up to that. Trust your friend to know what that means, and trust that they’ll still love you once you don’t know each other any more.