Does your library subscribe to Berg Fashion Library (online)? If so, it'd be worth looking up a few terms: dandy and flaneur.
In Berg Dictionary of Fashion History via Berg Fashion [online], I looked up the term, "dandy," which gave me a good definition of the term plus related terms.
Here's a copy of the definition:
Dandy
Related Entries:
heavy swell
masher
Source: Berg Dictionary of Fashion History
(M)
Period: ca. 1816 onwards.
A name for an exquisitely fashionable man, as represented by Lord Petersham. ‘The made up male doll who, when wig, dyed whiskers, stiff cravat, padded breast, paint and perfume are taken away, sinks into nothing.’ (The Hermit in London,ed. 1822) By 1829 ‘Dandy has been voted vulgar and Beau is now all the word’. (Disraeli, The Young Duke.) Count D’Orsay was described as ‘The last of the Dandies’ whose logical heirs were the heavy swell of the 1860s, and the masher of the 1880s and 1890s.
Also, though it's not listed as a "related term" in Berg Dictionary of Fashion History (perhaps your library has the print version of this title, if not subscribing to Berg Fashion Online?), I know from art history courses that another related term is the french, "flaneur." Indeed, the Berg Dictionary of Fashion History equates the flaneur to the dandy is a couple articles it contains.
Karen, I believe that if your patron seeks articles and books that discuss when & why men's fashion changed from being ornate to comparatively mundane, the patron will better understand why the dandy came about (probably in part as a reaction to the boring suits that became the standard dress for men, I believe as a result of the Industrial Revolution). Understanding how the Industrial Revolution gave rise to the city, and that respectable women could now go into public unaccompanied, informs how the "flaneur," the well-dressed, wandering lone male in Paris, came about. Impressionism deals with this topic.
Berg Fashion Library (online) offers images, but I'm sure ARTstor has images of dandies & flaneurs & early 20th century fashionable men, too.
***************************************************************
Hi Karen, the terms "flaneur" and "dandy" or "dandies" have come up a lot when I've been assisting students with this kind of question. Here are a couple of resources we have that seemed helpful:
Fillin-Yeh, Susan. Dandies : fashion and finesse in art and culture. New York: New York University Press, 2001.
Hollander, Anne. Sex and suits : the evolution of modern dress. New York: Knopf, 1995.
McDowell, Colin. The man of fashion : peacock males and perfect gentlemen. New York: Thames and Hudson, 1997.