Maybe they enforce RI through the front end. They might even do a great job on enforcing RI in a clear and consistant way. I still would never want to work on a database like this. But on the plus side DML run against the database should be fast. Especially inserts. Do they believe in indexes? They should be doing daily backups if they want to keep their data. and running in archive log mode is worth looking at.

IMHO if it was my database I would start at the top and add the RI a little at a time. I can't imagine why you would want to rely on the front end to enforce ri. One programming bug could corrupt your entire database.