This woman probably went back home and sat her ass on the couch. Or to a job just like the barista's. In my experience, those who feel the need to berate service staff are insecure, ineffectual and have zero going on in their own lives. They have no power over anything at all and try to snatch a little bit of it where they can and where they're least likely to face repercussions.

PS, I am one of those that does intervene. I worked in customer facing positions before going to law school, so I get it.