Read only/Standby database - Set user permissions Example database: WarehouseDB
Example user: reporting
Database state: Read only/Standby mode

How do I give the user 'reporting' access to only WarehouseDB without giving them sysadmin privileges? 

The problem occurred when I restored a bak file to create WarehouseDB in norecovery mode and then restored another transaction log file to put it in read only/standby mode for doing select queries for reporting purposes. And the reason I do log shipping then onwards is because both production and warehouse database can be synced at all times without any manual intervention. Now if I give this user sysadmin permissions then of course they can query the database but due to security concerns, I can't allow them. Also because it's a read only database, I can't create another user either and map them to WarehouseDB.

Another thing I forgot to mention is, the source database name is for example 'ProductionDB' and when restoring it's changed to WarehouseDB because I have mirroring setup for ProductionDB on the mirrored server where WarehouseDB is then restored.

Any ideas on how to resolve this?
The user has to be granted permissions in the source copy of the database.

The restored copy is just that -- a copy -- of the source, so there's no way to change one and not the other.

Since this is a SQL Server login, you'll need to create the logins on both servers with identical SIDs so the mapping from the database user to the login matches in all cases. You can do this by recreating the login on the secondary, specifying the SID manually in the CREATE LOGIN statement.

If you went with a solution such as replication (not saying you should, just giving an example), this works using two read/write databases, where you could grant permissions completely independently.