How to view the original script that built a materialized view and that was created by another user?
Specify the owner in GET_DDL, or use ALL_MVIEWS instead of USER_MVIEWS:

select dbms_metadata.get_ddl('MATERIALIZED_VIEW', 'MVIEW_NAME', 'OWNER') from dual;

select query from all_mviews
  where mview_name = 'your materialized view';