ORACLE- Database unable to START/OPEN
Generally you can open standby databases as read only. Specifically you can open data guard standby databases as read only. Did you check to see if all of the data files exist? That a database is mounted, means that the control files are where they are supposed to be, not corrupted and your init/spfile is valid. 

Did you patch the Oracle Home for the standby database with the same patch as the parent? Can you force database recovery on the child to get the databases in sync.