Okay, let me see if I get this. It's "sall" because I'm talking about my friend's movement away from me. If I were talking about my friend's existence over there, it would be, "Bhí mo chara thall anuraidh." Is that right?
Yes — sall is a direction (“thither”), and thall is a position (“yonder”).
This whole idea of going over, the Irish Diaspora as it were, strikes me as something Tolkien might have borrowed for his story ending with Elves sailing to the lands of the West.