[autarch]

The "R" stands for "relations", as in "relations", which is a mathematical concept. SQL calls a "relation" a "table". The "relational" is RDBMS has nothing to do with relationships.

But I still agree that OP's colleague is an idiot.

[hu3]

That might have been how it started (https://www.ibm.com/ibm/history/ibm100/us/en/icons/reldb/).

But it's definitely not what it means for the great majority of contemporary contexts.

Relations in modern RDBMS are usually aliases to foreign keys unless otherwise specified.

[goto11]

To be really pedantic, tables are relations but a join between two tables are also a relation. Base tables, queries and views are all relations and therefore interchangeable in relational algebra.