First you need to be clear on what SQL is and is not. SQL is a query language and data manipulation language used for accessing and manipulating data in a relational database. But the schema and data objects (tables, columns, indexes, constraints) in the database are not "in SQL", SQL is just one possible language for querying and manipulating the data.
In order to be able to work effectively with a relational database, you need to understand tables, columns, datatypes, primary keys, foreign keys and indexes. You also need to understand the basics of querying: projection, filters, joins. You need to understand the basics of normalization.
But none of these things in principle requires you to touch SQL. You may be able to design the database schema in a GUI designer, and you may be able to write queries and updates in some other language like SqlAlchemy for Python or Linq for .net. Some even argue these languages are a purer representation of the relational model than SQL.
So in theory your friend is right - you don't need to learn SQL. But you still need to learn how relational databases work, and when you know that, SQL is pretty easy to learn, since it is just some syntax.
While not necessary, it is quite convenient to know SQL, since you can query any database directly in SQL without the need of a separate translation layer. And since all tutorials, books and examples use SQL, it will be difficult to avoid learning it.