Why is a comma used in the following sentence?

In the following sentence Mary was working at Pete's Cafe, where she was the only female, is where she was the only female a nonessential clause appropriately set off by a comma? The general agreement in my writing class was that the comma in this sentence is actually used informally-and perhaps ungrammatically-as a kind of breath mark; the body of text the sentence came from was an example of informal writing. What am I to think?