I can't flag it as a duplicate, because the other question is on a separate site. Do I answer it? Do I direct to the other question in a comment? Do I try to attract a moderator's attention some way?
Do I answer it?
If the author has provided enough details then you should answer the question.
Do I direct to the other question in a comment?
If the question is answered on Server Fault you could vote to migrate it to Server Fault. As a user on Server Fault you should then vote to close the question as a duplicate. You should not leave a temporary comment linking to a possible solution.
Do I try to attract a moderators attention some way?
The only thing a moderator could do it migrate the question to another community or close the question, if there is nothing wrong with the question, then you should either flag it to be migrated or answer the question.
In this specific case the question, you are asking about, is not eligible to be migrated.