They cant and normally the Germans have other things to do than just sit in southern Austrian countryside.
Actually, here is the exact rules for that:
Italy rule:
Italy is only at war with Austro-Hungary when they enter the game on turn 3.�  They do not go to war and attack the other central powers until turn 6, although Italy may be attacked by any of the central powers and going to war early with all of them if that happens.
German units cannot enter Austria-Hungary till turn 6
Not so historical when you think of the german Alpenkorps. Although Germany and Italy were not at war until 1916, the Alpenkorps was immediately dispatched to reinforce the thinly occupied front line in the Dolomite mountains. It did not undertake offensive actions, but defended the front against repeated attacks by the Italian Alpini until Austria was able to extract enough forces from the eastern war theatre and relocate them to the new front.
It would be better when German units cannot enter Italy till turn 6 but why shouldnt they help prevent the collapse of Austria Hungary? Historically german units helped their austrian buddies on Eastern front in Galicia in 1915 already, think of Mackensens 11th army at Gorlice Tarnow (located in Austrias Galicia)