Below is an sample script you can tweak for your needs.

IF OBJECT_ID(N'tempdb..#GuidColumns') IS NOT NULL
   DROP TABLE #GuidColumns

DECLARE
   @GUID uniqueidentifier,
   @TABLE_SCHEMA sysname,
   @TABLE_NAME sysname,
   @COLUMN_NAME sysname,
   @UpdateStatement nvarchar(4000)

-specify uniqueidentifier value to find
SET @GUID = '00000000-0000-0000-0000-000000000000'

SELECT
   TABLE_SCHEMA,
   TABLE_NAME,
   COLUMN_NAME,
   CAST(0 AS bit) AS Found
INTO #GuidColumns
FROM INFORMATION_SCHEMA.COLUMNS
WHERE DATA_TYPE = 'uniqueidentifier'

DECLARE GuidColumns CURSOR
   LOCAL FAST_FORWARD FOR
   SELECT
       TABLE_SCHEMA,
       TABLE_NAME,
       COLUMN_NAME
   FROM #GuidColumns
OPEN GuidColumns
WHILE 1 = 1
BEGIN
   FETCH NEXT FROM GuidColumns INTO
       @TABLE_SCHEMA,
       @TABLE_NAME,
       @COLUMN_NAME

    IF @@FETCH_STATUS = -1 BREAK

   SET @UpdateStatement =
       N'UPDATE #GuidColumns SET Found = 1
       WHERE
           TABLE_SCHEMA = @TABLE_SCHEMA
           AND TABLE_NAME = @TABLE_NAME
           AND COLUMN_NAME = @COLUMN_NAME
           AND EXISTS(
               SELECT *
               FROM ' + QUOTENAME(@TABLE_SCHEMA) + '.' +
               QUOTENAME(@TABLE_NAME) +
               N' WHERE ' + QUOTENAME(@COLUMN_NAME) + N' = @Guid)'

   EXEC sp_executesql
       @UpdateStatement,
       N'@TABLE_SCHEMA sysname,
           @TABLE_NAME sysname,
           @COLUMN_NAME sysname,
           @Guid uniqueidentifier',
       @TABLE_SCHEMA = @TABLE_SCHEMA,
       @TABLE_NAME = @TABLE_NAME,
       @COLUMN_NAME = @COLUMN_NAME,
       @Guid = @Guid

END
CLOSE GuidColumns
DEALLOCATE GuidColumns

SELECT
   TABLE_SCHEMA,
   TABLE_NAME,
   COLUMN_NAME,
   Found
FROM #GuidColumns
WHERE
   Found = 1
GO

foreign key. But there is no relation defined to what table this
foreign key refers [I think you mean that you have no REFERENCES
clause?]. I suppose the link [sic: reference] between these two tables
is purely managed programmatically. <<

I worked for a company that wrote crap like that when I first moved to
Austin. They are still in business, but down from ~1200 employees to
~200 and are now outsourcing their Indian operations to China as they
shrink.

This kind of thing falls apart in about a year.  You get orphans all
over the schema (we choked a hard disk for a relatively small custom
sales commission package).  There is no ways to validate or verify a
GUID; you need a proper key instead of a bad attempt at pointer
chains.  The "pseudo Foreign Key" will be referenced by Cabbages and
Kings simply because all GUIDs can be compared (like pointers or other
exposed physical locators).

You really need to throw this thing out.  But if you cannot, then
update your resume.