Were do we stand

My brother bought a late model low klms Ducati Hypermotard from a well established Ducati dealer (not local to him) a few months ago.
He paid a premium for it but thought it was better to buy a bike from a dealer than privately.
Forward about 6000km and he put in into a local bike mechanic to get a service done and they call him to say who last serviced your bike as it has NO AIR FILTER
He accepts the bike was used and has no warranty but I think there is a duty of care here on the selling dealer to check the bikes over before they put them up for sale.
Question is where does he go from here?

Sure there is no filter as it sits in a funny place!?

I think legally this a funny one as it's going to be hard to prove that nobody touched the bike for 6000km and the filter wasn't there.
I would ring them and politely try the if you don't fix it (full refund or so) I'll do the name and shame to the importer, internet and all.

Definitely no filter fitted.
In NSW used bikes are cover by a 3000km and or 3 months warranty when purchased from a dealer.
The motor is sure to be dusted as he has done a few dirt roads.