We want to provide Microsoft SQL Server 2012 Enterprise licenses to a hosting provider who is to run the licenses in a cluster with an active and a passive server. How do we need to license this scenario?

When reallocating Microsoft SQL Server 2012 licenses to a hosting provider, these licenses need to be covered by active Software Assurance (SA). Licenses can then also be reassigned as often as desired within a server farm. There is no need to purchase licenses for the passive SQL Server, as long as it is running in a Operating System Environment (OSE) separate from the active SQL Server.

Sources:

Licensing reference guide_Microsoft_SQL Server 2012_as is 2018-08-29
Microsoft Product Use Rights, August 2012