There's nothing wrong with ...f5
I just want to mention these are some easy ways to play against it. First is like a standard queen's gambit type setup. Second is like a reversed london. Both involve natural development and will give you a playable (even favorable) middlegame where you can just play chess so to speak. In other words no specialized knowledge needed.
I'd like to play 1.f4 f5 as black but looking at a database it seems so rare that I'm concerned there might be something wrong with it. Is it good? It seems hard to believe there could be anything wrong with responding symmetrically to white's move.