If his auction says - the only one for sale on eBay - and that is what you want to report, I wouldn't bother really.
If people want one of whatever it is you are both selling then they type it in the search area and both your auctions will come up. If he has used your photos or something then you have something to report, but I don't think you can report his auction just for making a mistake, sorry.
i don't see why it matters....