We have a SQL Server 2014 Enterprise Edition (30 cores) production database. We're using Always ON to provision a read only replica. Does the read replica instance also need to be Enterprise Edition with support for 30 cores?
From the licensing Link (PDF)
The secondary server used for failover support does not need to be separately licensed for SQL Server as long as it is truly passive, and the primary SQL Server is covered with active SA. If it is serving data, such as reports to clients running active SQL Server workloads, or performing any “work”, such as additional backups being made from secondary servers, then it must be licensed for SQL Server.
Make sure you do a 1-1 matching between Primary and Secondary replicas since in the event of failover, your secondary (now primary) should be able to handle the same workload as the old primary.
Note: Since you have 30 cores, make sure that you are utilizing your resources and not having your cores run idle .. time to do some consolidation if possible.