You’d have to use an event. A warning will not stop the process, which an error will. Which version of IFS is this for? 
 

Below will produce a warning/info message. There is one called Add_Warning, but that doesn’t work on ActiveSeparate.

Client_SYS.Add_Info('LogicalUnit','TEST: This will be my information message' );

Below will produce a stopping error message.

Error_SYS.Appl_General('LogicalUnit', 'ERROR: This is the error message I want to display');

So, maybe something like this:

DECLARE

   CURSOR get IS
      SELECT cf$_your_field cf
      FROM   active_separate_cft
      WHERE  rowkey = '&NEW:ROWKEY';

BEGIN

   FOR rec_ IN get LOOP

      IF '&NEW:WO_STATUS_ID' = 'FINISHED' AND NVL(rec_.cf, 'FALSE') = 'TRUE' THEN
         Client_SYS.Add_info('ActiveSeparate', 'TEST: Your information message');

      END IF;
   END LOOP;
END;

DECLARE

   CURSOR get IS
      SELECT cf$_your_field cf
      FROM   active_separate_cft
      WHERE  rowkey = '&NEW:ROWKEY';

BEGIN

   FOR rec_ IN get LOOP

      IF '&NEW:WO_STATUS_ID' = 'FINISHED' AND NVL(rec_.cf, 'FALSE') = 'TRUE' THEN
         Error_SYS.Record_General('ActiveSeparate', 'TEST: Your stopping error message');
      END IF;
   END LOOP;
END;