What is ‘He who becomes a sheep is eaten by a wolf’ in Ancient Greek?

There have been some changes between Ancient and Modern Greek: τρώγω originally meant “chew” (it’s the same evolution as Latin manducare > French manger). By Attic, πρόβατον meant sheep and not just livestock; the Homeric word is ὄις. And ὁποῖος is “of what sort”, not quite the proverbial “whoever”.

I haven’t studied Classics at university either, but I’d suggest:

ὅστις πρόβατον γίγνεται, λύκος ἐσθίει αὐτόν.