This is all very interesting. I think that a lot of people take a certain level of racism or anti-semitism for granted, while not overtly judging someone for their race or religion, they still don't see the problem in the language. "Jew Couple" is wording with roots in anti-semitism, and even if the waitress didn't know any better, she should learn that those words have emotional baggage. Jew is a noun, Jewish is the adjective. So Jewish couple would not be so emotionally loaded.
However, the anti-semitism part here is underlying everything. Unless the couple self identified as being Jewish, and that had some bearing on their food order, then the waitress had no business using that descriptor. An example of a case where it does make a difference; my sister does catering, and if she has a lunch to do, where sandwiches are served, she will ask if there are any Jewish guests. And if there are, just to be safe, she will separate the meat and dairy, just in case. That would make it useful to have "Jewish couple" indicated on the food order, that means, don't serve them the ham and cheese sandwich with butter and mayo. (She also asks about other religious food restrictions, and other dietary issues.)
This whole issue really came home to me years ago. I am left-liberal, raised in a family that had many many friends from many parts of the world, we learned to embrace different cultures. I thought I was as racially "blind" as you could be. But I was in a university graduate school program with students from all over the world. We were at a bar on a Friday night, early in the year when we didn't all know each other, and one of the professors was trying to find out the name of one of the students. He kept asking, what's that guy's name, The guy standing over there, wearing a blue shirt, he's about my height, etc. etc. Finally I realized who he was referring to, and said, oh, you mean the Black guy? (He was the only African American in the bar at the time. One black face in a sea of white). The professor looked sort of confused for a minute, then surprised, and said, oh, right, he is black, isn't he! The prof had just arrived from teaching in Tanzania, and said that in his first day there, realized that identifying someone as "that black guy" just didn't give any useful information. And after several years, literally did not notice someone's skin colour.
That is true racial blindness. The fact that the waitress noticed and identified the couple as Jewish is still exhibiting remnants of anti-semitism.