The problem here (and I’m sorry if I’m misunderstanding robberfly) may not be so much “first citation”, or “coinage”, which as Dave W says, probably can’t be determined with a phrase like this, but “when did the phrase start to become popular”, which is a different sort of investigation.
Using the rather crude investigative probe of Amazon.com’s “search inside” facility turns up one 19th century reference, from The physiology of New York boarding-houses by Thomas Butler Gunn, published 1857: “… its dinner-hour fashionably late.” Then there is a huge gap until 1971, and Hunter S Thompson, of all sources, who apparently wrote in Fear and Loathing in Las Vegas (p108, 1995 edition - I can’t confirm it’s in the original because, I’m ashamed to say, I don’t have it on my shelves, though I read it in Rolling Stone way back then): “those of us who signed up fashionably late were assigned to the Flamingo”.
This is followed by a six-year gap until 1977 and Slow Fade to Black by Thomas Cripps (p364): “… She pulled it off in the Hollywood style, fashionably late, wrapped in ermine, gardenias in her hair, seated at Selznick’s ...” Then there is another gap of 10 years, until 1987, and Wife of...: An Irreverent Account of Life In Washington by Sondra Gotlieb, page 94: “… The next night we come fashionably late. The outside guests have arrived, wearing silk dresses.”
A two-year hole then follows to 1989, after which there is a constant stream of references - more than a thousand - through to the present day, including Olivia Goldsmith, author of the novel The First Wives Club, who called her second novel, published 1993, Fashionably Late, which suggests that the expression was well-known enough in the early 1990s to be chosen by a popular author as a title (there have been a couple more novels called Fashionably Late since then). and the great Linda Thompson, who was able to call her album released in 2002 Fashionably Late as a jokey reference to the fact that it was her first release for 17 years – that’s not really relevant, I just wanted to mention Linda Thompson …
Now, those dates, and gaps, may be an artefact of the inadequacies of Amazon.com and “search inside”. However, they suggest a phrase that was “in the air” in the 1970s but never became mainstream until the very end of the 1980s, and which which has hung around ever since … is that any help, robberfly?