It's chapter 686. The page:
Zoro doesn't mention haki. Monet was targeting Nami while she was carrying a Chopper who couldn't move after Monster Point, hence why Zoro lectured her on attacking the weak and vulnerable.
It is pretty much undeniable that heat did work against Monet, in my opinion. If Nami explicitly states that heat works and Monet screams in pain because of it, then she's probably hurt beyond her body snow. Otherwise, what does even mean for it "to work" if it's just melting snow with no other effect like any other non-haki technique would do? Nami even tells Monet to better keep her distance —why, if heat balls are useless?
As I said, I agree that the Magu Magu doesn't counter the Hie Hie nor vice versa. But let's take into account that, if heat does hurt Monet's Yuki Yuki, then she will be probably damaged by the Mera Mera fire alone. And, considering that the Magu Magu is inherently better than the Mera Mera while the Hie Hie is inherently better than the Yuki Yuki, Oda could easily defend that Magu Magu > Hie Hie > Mera Mera > Yuki Yuki in terms of elemmental interaction. Interestingly enough, Ace's fire clashed equally with Kuzan's ice in Marineford while Sakazuki's magma engulfed the former's fire and melted with ease the ice Jozu threw at him.
Again, I don't think this will be the case because they're both top class logia and experienced users, so Kuzan can probably reach a temperature low enough to cool magma down (and the other way around). But I don't think it's as obvious as this thread seems to make it.