First, it is important to understand that the "friends" of Italian unification (other than those in Italy) were friendly to the cause not because of any kind of philosophical reasons but because they thought that Italian unification would be good for them.
The main "friend" of a unified Italy was France, which was then ruled by Napoleon III. France thought that a unified Italy would be a good ally against Austria. Therefore, France set out to help Piedmont in a war against Austria. Prussia was also, to some degree, a friend of Italian unification. The Prussians, too, were rivals of the Austrians. When Italy (by now a country) allied with Prussia in a war in 1866 against Austria, they were rewarded with Venetia and Rome, thus completing the bulk of the unification.