'¿Han cocinado en esa cocina?' Does not the use of 'han' mean 'them' - or if it's 'you-plural formal' does it not need ustedes in the sentence? Or am I missing something very obvious?
You don't -need- ustedes per se, it just helps to clear up any confusion that may occur in an ambiguous scenario. Anyway the answer they gave is "has" not "han", meaning spoken to one person (tú)
"Han" should be accepted though shouldn't it? I haven't tried it myself - so it might be, but it should work as far as I can tell to mean "Have you (all) cooked in that kitchen."
Reported "han cocinado" (they not we)
anyone else see 'have you cooked that kitten' at first?