If i move Irresistible Faefolk to battlefield and move enemy unit to my battlefield, my enemy react with Gust then i react with Rengar, Trophy Hunter. can my enemy play rebuke from their hand to return my rengar

No — not during that chain. Rebuke is an [Action] spell ("play on your turn or in showdowns"), and while your Faefolk trigger, their Gust, and your Rengar are still on the chain the game is in a Closed State, where the non-turn player can only add [Reaction] cards (FAQ #3425).

So they can't answer Rengar directly. What they can do is wait: once the chain empties and Cleanup stages a showdown at that battlefield (both your Rengar and the pulled enemy unit are there, so it's a Combat Showdown), the state goes Open and both players get Action/Reaction windows before damage. In their Action window — even on your turn, because it's a showdown — they can play Rebuke and return Rengar to hand, leaving their unit alone at the battlefield.

One thing to check on your side: Rengar, Trophy Hunter's Ambush permission requires enemy units to be at that battlefield at finalization (Rule 822.3), so he's only a legal play there if the enemy unit is already present — the Faefolk trigger pulling them in doesn't count until it resolves.