RMAN restore of primary database - impact on standby database
You should be able to use Flashback Database to get the standby back to the point in time (i.e. SCN) to which the primary was restored (Flashback Database is a requirement for automated failover).  If you can do this, then Data Guard should be able to wire things back up together again.
Have a look at these:

http://www.br8dba.com/resetlogs-on-primary-where-standby-in-place/
http://oracle-help.com/dataguard/flashback-database-after-resetlogs-in-data-guard/

If in doubt, though, or if you don't have Flashback Database enabled, then trash the standby and recreate it from scratch.