Upvote:2
Your friend should be able to transit in Athens from Sofia to Hong Kong, provided that she is traveling on a single booking such that she doesn't need to pick up and re-check baggage. (She would need to enter the Schengen are in order to reach the baggage claim).
Traveling on a single booking, she should get boarding passes for both legs when checking in in Sofia; there's no need to visit a check-in desk at Athens.