Peter Jackson posted on Facebook:
I am sad to report that Rob Kazinsky, who was cast in the role of Fili, is having to leave The Hobbit and return home, for personal reasons. Rob has been terrific to work with and his enthusiasm and infectious sense of humour will be missed by all of us. I should say that Rob’s departure will not affect ongoing filming of The Hobbit, nor will it impact work done to date, as we had yet to film much of Fili’s storyline. At the moment we are shooting scenes featuring Bilbo without the Dwarves, which will give us time to find a new Fili. I’ll keep everyone posted with updates as they come.
So although it is a shame to lose Kazinsky, it sounds like it really won’t affect the production schedule.