na czyjeś niesczescie

jak powiedziec np

Na jego nieszczescie trzeba miec 18 lat.

ale inaczej niz unfortunately

misfortune? ;)

It's his misfortune that he isn't 18.
It's his bad luck that he isn't 18.
Unfortunately for him, he isn't 18.
It's unfortunate for him that he isn't 18.