Why not? All this brouhaha wouldn't have happened without their paperwork, their receipt and most importantly their warranty service. I can bet you that they would probably never bother to turn up and give evidence if they weren't a party to this whole thing. Anyways, the most important part here is that Nikon has denied warranty because of previous reports regarding the lens and quoted quite an exorbitant price to repair it. At least you can ask them directly why that is so.

The tribunal can then rule whether or not Nikon's action in denying warranty is correct or not in the circumstance, whether it is obligated to assume warranty under the circumstance, why the repair fee seems to be more that the retail price etc etc. Heck, sitting here I'd probably be able to dream up a multitude of questions I want to query Nikon as well, such as what happened when warranty was denied in the first place.

After all, buyer bought the lens under the impression that Nikon is providing the warranty, right? Did Nikon provide a way to check warranty? Is warranty voided by change of ownership.

Don't forget, the Tribunal's role is resolving what is fair. Do you think that repair bill is fair? Yes? No? Maybe?

Let me just give you an example of my car's story. I didn't win because of what it is the warranty booklet. I won because the Tribunal ruled that the car sold to me WASN'T UP TO QUALITY, as per the Consumer Act, regardless of warranty.