When did "Son" become a racist comment?

so I am outside of a customer's office this morning, and there was a guy outside that was looking for his last paycheck. He had been recently fired.

The owner walks outside and stops because he hears the rukus (can you describe the rukus?). He says, "...Son, just calm down, you will get your check."

Upon hearing this, the wife loses her sh!t, and screams that, "..."son" is racist name, you can't say that!"

This must have become a racist word when I wasn't looking??