Is it critical for a database to have a relationship table/diagram or not? Got a question and wanted to get the best practices or thoughts on this.

I have an application that was developed with a SQL Server back end (site is Entity framework, vb.net, asp.net). The original design does not have a database diagram/schema/relationship table in the db itself.

The site works great but have gotten some pushback from another group that says it has to have a relationship table in order to work properly.

So is this personal preference or what? I'm not a DB guru and trying to find out if it's two ways of development and someone is trying to push personal preference over standards or best practices.
You don't need a diagram for it to work, you need one to train the future developers!

However, if what you really have is a database with no actual formal PK/FK relationships set up (Which you can do when creating diagrams as well as through better ways such as SQL scripts), then that is a different story. That is something that will eventually cause your data to be inconsistent and is a real problem indeed to other users of the database.