Manually highlight and run #1. Paste the result into #2.
-- DropFailedForUser.sql
-- The statement (that caused the error)
USE [master] -- in this case
GO
DROP USER [Dom\SomeUser]
GO
/* The Error ...
Drop failed for user 'Dom\SomeUser'
The database principal owns a schema in the database
and cannot be dropped. Error: 15138
*/
-- #1. find the name of the schema
SELECT [name]
FROM sys.schemas s
WHERE s.principal_id = USER_ID('Dom\SomeUser');
-- #2. transfer ownership of the schema to 'dbo'
ALTER AUTHORIZATION ON SCHEMA::[SomeSchemaName] TO dbo;
-- repeat "The Statement"