I seem to recall that you don't need to license a passive replica. At least for Enterprise.
However, this is a technical forum, and this not a place where you can get authoritative answers about licensing. You need to contact your local Microsoft representative.
You can also call 1-800-426-9400, Monday through Friday, 6:00 A.M. to 6:00 P.M. (Pacific Time) to speak directly to a Microsoft licensing specialist.1