Terrible situation for the OP.
I would suggest that he, as suggested by others, contact both the Finnish embassy as well as Finnair. (maybe via social media?)
It should be possible to find him a solution - either by allowing him to enter Schengen a day earlier (or better said, couple hours) then when his Visa starts, or by providing him with a letter that he's allowed to board the flight to HEL and wait in international (non-schengen) area till midnight.
Hopefully their will be a solution found. It's - luckily for him - not Germany "zis is a rule"-country but Finland, where common sense haven't totally be lost.