Hi,
It depends on how the author is stored. I tried to check the back-end, but all links you sent lead me to a 404 error page.
I recommend maybe trying to enable the search in terms option. If the author is stored as a taxonomy term, then that should help. Maybe even trying searching custom fields, in case the author is stored as a meta field.
If by the author you mean the actual post author (who published the post), that could be doable as well via the index table engine. When configuring the index, you can choose to index the author data as well.