Here's my current booklist on everything that can be construed as "mid to late 19th century", I'll list it entirely for your future use but I'll recommend some at the end.

• Carr, William, The Origins of the Wars of German Unification

• Badsey, Stephen, The Franco-Prussia​n War 1870-1871

• Herwig, Holger, Politics of frustration: The United States in German naval planning, 1889-1941

• Blackbourn, David, History of Germany, 1780-1918: The Long Nineteenth Century

• Blackbourn, David & Eley, Geoff, The Peculiarities of German History: Bourgeois Society and Politics in Nineteenth-Cen​tury Germany

• Massie, Robert, Dreadnought

• Weber, Eugene, Peasants into Frenchmen: The Modernization of Rural France, 1870-1914

• Howard, Michael, The Franco-Prussian War

Now, considering this is what I assume is your friends second book, and he really likes WWI history it seems, Dreadnought is a wonderful choice in my mind. His other work, Castles of Steel, is more focused on WWI itself while the former is more on the arms race that led up to it as well as acting as a generalized solid work on how the war started. William Carr's work is a small little handbook but it remains to this day my favorite book of any in any subject. It's just spectacularly well written and concise and your friend can finish it rather quickly and leave with a very well versed understanding of German nationalism and German 19th century social history. However, be warned, that's more "early" 19th century than anything else and breaching into the "mid" near the end. Blackbourn and Eley's work, Peculiarities, I'll just say is dense but probably the most important book of Germany written in a long time. I think the reading can sometimes take 2 or 3 reads some pages but, as an amazon review put so wonderfully,

The standard question for many years in European history was, "Where did Germany go wrong?" The standard answer (in different forms) was "Germany somehow deviated in its development as a modern, liberal-democratic capitalist state." Blackbourn and Eley (in slightly different approaches) ask the bold question, "Was there any state that followed a 'normal' path of liberal-democratic capitalist development?" The short answer is no, and if read carefully, this book turns inside out a standard narrative of Western Europe (i.e. after the Enlightenment, states developed into capitalist states which encouraged the growth of liberal-democratic governments and a fading of the traditional noble class by the end of the nineteenth century). Blackbourn and Eley, with careful use of evidence, point out that this 'normal development' never actually occurred anywhere. Any book you read by contemporary historians of Modern Europe owes an intellectual debt to this text. Sure, it has some flaws, but they are few, and these two essays have stood the test of time.

So I would recommend this work to him but put it with a caveat of it should come after you have read somewhat of European history in this period as it is assumed you know it. Lo and behold, Carr's work is just the thing for an introduction to social history in this time period! :) Beyond that, Howard's Franco-Prussian War is also a wonderful book I could recommend for his next work too.