[astura]

Your colleague doesn't understand relational databases, which, unfortunately, is fairly common - classes on RDBMSes are electives in most CS programs. It's scary/concerning this person is being allowed to operate like this - I'd suggest your company add design reviews and code reviews to reel this sort of rouge behavior in.

...Of course, if nobody understands relational databases at the company, then that won't help...

Even if data migration was difficult with foreign keys for some reason, they can be disabled at the time of migration - no need to put weird constraints on day-to-day operations.