I thought this was one of the sillier recent World War I "what ifs", and an aristocrat believing it was possible does not make it less silly.
Italy was pretty far down on the list of important World War I fronts, and while throughout the war there were proposals floated on both sides that making a big push there could accomplish something, the basic facts of geography always prevailed. There also doesn't seem to have been any plans in Vienna to restore any part of the Hapsburg hegemony over northern Italy. And in the event they didn't get past the Piave.
So the only way you can get something like this is a complete internal Italian collapse. Treating this as a AHC, the only way I can get something like this to happen is something like this:
a) Italy joins the losing side (either one)
b) Italy does poorly in the war and sues for peace, about the time that its allies do
c) A bolshevik revolution happens. Maybe Mussolini tries to seize power and chaos erupt
d) The winning coalition in the war sends forces into Italy to stabilize the situation and get a co-operative government out of it
e) A south Italian separatist movement sides with the intervening forces
f) The central Italian government in Rome remains bolshevik or whatever, but the Allies/ CP settle for breaking off parts of the country and installing friendly governments
To get this I've used OTL Turkey as my template, borrowing liberally from OTL Russia. I'm assuming a neutral Italy won't work since they do too well trading with both sides.