The absorption of Austria as Prussia requires that 1. Hungary be independent and 2. Austria be knocked out of GP status.
#1 could likely be the player forcing Austria to release a 1-state Hungary for this. #2 is quite a bit harder to do, though, on par with Austria knocking Prussia out of the same. IMO its marginally easier to get Greater Germany as Austria than Prussia, though simple unification is much simpler as Prussia.
This player, if he did it legit in 5 years, knows a trick I dont about knocking Austria down a peg.
I assure you no trick is included ok here is what happened in short: i start the game as usual building 10 brand new brigades and allying with denmark(influencing them along with holstein), then again as usual Austria jumps on Denmark, Denmark ask for my help and I start waiting austrian troops near my borders(leaving one province between)
When Austrians(or rather sheeps) started occupying my border provinces I attacked them choosing battles wisely and most importantly in crushing odds and they started suffering heavy losses(thanks to my "+attack" generals) and began retreating in disarray(love the word) and soon they were all but destroyed. I'd already added "assert hegemony" war goal after that i also added "free people-hungary"
then started a long but fairly easy occupation of austria and i called all my german minor allies and after almost 2 years occupation was completed but the problem was that Austria was still the 7th GP but only a few points above the Ottomans and Spain approximately six months later they dropped to 9th place and another six months saw them dropped to secondary power status THEN I added "ADD to SPHERE" war goal and they accepted the peace.
Now denmark was already in my sphere and I formed NGF built another 10 new brigades and declared war on France and applied the same tactic which was wait for them to come-defend-beat them-chase-beat them again and again and again and again with superior numbers(don't worry if they occupy luxemburg etc.) while your allies(bavaria, baden and wurttemberg) occupies france and then after a short while they offered peace giving me alsace-lorraine
and then came the hardest part:with austria safely in my sphere, hungary free, got in politics, clicked on the tick next to three hurrah for germany and Großdeutschland