I'm really trying to be okay with everyone calling this beef dip "au jus" (as in "you forgot the au jus" or, even worse, "served with au jus"!) but... I'm struggling to overcome my own prescriptivism...Well, in a fit of procrastination, this made me start to think about what sorts of mangled uses of borrowed words and phrases that native English speakers blithely use without batting an eye. I mean, I understand the thought process that my friend is having, since I sometimes have similar fits of, "You just can't... That's grammatically wrong!" for something that is becoming linguistically commonplace and - by consensus - correct.
My particular pet peeves come with words that have been uncountable while I have grown up, but which have - increasingly - become countable. Things like "jeans" and "pants" and "trousers." As adjectives, "pants" and "trousers" - like most noun-adjectives in American English - lose their "-s":
However the "jeans" remains with the "-s":
Why? Because of social conventions in the form of American English that I grew up hearing. However, the trend of transforming "pants," "trousers," and "jeans"from uncountable (if plural-looking) nouns into countable nouns is really ... annoying to me. For example, the sentence, "This is a really good-looking pant," doesn't make me think of an article of clothing, but of someone, somehow, managing to make panting look good.
But these uses are (for some reason) becoming more commonplace. At least in retail in Arizona. And to these young women who are talking about a "good-looking pant," it is likely that they don't see anything wrong with their use of the word, despite historical norms. Why? Because the historical English is not the one they grew up speaking.
Which then leads me to the question of how much our linguistic prescriptivism is determined by the fact that we are prejudiced to prefer the type of English that we are most comfortable with, and all the hand-wringing done by previous generations of linguistic prescritivists amount to less than a hill of beans.
For example, I use "kudos" as plural, despite it originally being a singular noun derived from a singular Greek noun.
I also use pluralize the many nouns borrowed from Japanese (like "samurai" and "katana"), despite Japanese having no general grammatical concept of plural vs. singular (and only making things explicitly plural when needing or wanting to emphasize the plurality or collectiveness of that thing). Similarly, I tend to think that people who really try to hold to not using plural forms for any Japanese words to be really pretentious, since - in English - the only real alternative is to make the nouns uncountable, which doesn't make grammatical sense when speaking in English, except when those nouns fit into general categories of uncountable nouns (e.g., "sushi," "sake," or "jujistu").
So I'm not really that lingustically presciptivist when it comes to grammar of borrowed words. However, it does irk me with words whose meanings get twisted too much when brought into English. For example, the word "geisha," which describes a female entertainer of traditional training in traditional Japanese performace art. However, the social implications surrounding women putting on performances for men comes from very different historical contexts when comparing American and Japanese understandings of the term. And in some way, this term - like many others that are embedded in and grow out from an embedded social understanding - is like any one of those terms that make the lists of "words that exist in other languages but not in English" that sometimes get passed around the interwebs.
But to return to the question of whether it is more "correct" to use "jus" or "au jus" (or even just asking for your "meat juice"):
There are many cases of knee-jerk reactions to linguistic change, but - unlike languages that have a "language academy" that exist as the "ultimate authorities" on all questions about a language's grammar and lexicon, the appropriate use of a term in English is basically up to a free-form, tacit democratic decision-making process. People get hung up on the changes to the implicit truths about English that they grew up hearing and using (and getting corrected over by people in authority over them, such as parents or teachers), while never coming into contact with the massive heaps of changes that occurred to the English language in previous generations (and over which prior generations likely had their worries over).
So, if "au jus" is becoming the implicitly understood term for "meat juice" (and eventually becomes rewritten as "oh-joo"), then that's okay. American English has done this a lot. One good example is how Americans have reimagined the etymology of "hamburger" to make the etymologically incorrect words "cheeseburger" and "cheeseburger" (and even going to such an extent of back-forming a term as to make the word "beef burger" to describe what was originally meant by "Hamburg steak").
We all have our hang-ups about what English "ought" to be. However, what looking into how English is used across the globe and has been changed over time has shown me is that English is constantly evolving, and - short of crossing the socially accepted boundaries of appropriate English usage in the group and setting and timeframe in which you are trying to set yourself - the linguistic hang-ups you have are totally your own.