Dave Bland walks us through one way to fix an orphaned user:
In my many years of working as a DBA, I have encountered many disabled logins. However, I have never really encountered what looks to be a disabled database user account. I didn’t even think it was possible to disable a user account in a SQL Server database. I checked the user account properties just to makes sure I was correct. Sure enough, no option to disable a user account. This finally turned out to be a simple case of looks can be deceiving.
You can also use the sp_change_users_login procedure to fix orphaned users.