Login problem querying by SSMS

Hi guys, I got one colleague, let's say Tom that is having problems connecting a db by SSMS. The situation for Tom is:

Security-->Login He has permission in the DB as datawriter

DB-->Security-->Users He has permission has datawriter

but when he try to run a query it retutrns the msg

I tried to delete the user (DB-->Security-->Users) and/or to remove the user from the db (Security-->Login) but both operation return The dataset principal owns a schema in the database, and cannot be dropped.

Any thoughts? Thanks