As long as the streaming replication is in asynchronous mode, you can restart the standby anytime without affecting the primary. You can check whether replication is synchronous or asynchronous with "select sync_state from pg_stat_replication;"
If it says 'synchronous', you will have to first change it to asynchronous before you can reboot the standby, or else all commits on primary will be pending waiting for the replica to replicate those commits first