It could easily be an improperly mounted tire, or a defective tire. I had that same situation in the past after having new tires installed, except instead of a wobble I had a weave (rear end moves side to side). If he wants to verify it's the tires and not anything else on the bike, one way to do that is to swap wheels and tires with another bike that doesn't have an issue. Time consuming I know, but I doubt any shop is going to just assume the tire is defective and replace it. And of course if both tires were changed, then they would still be guessing as to which one might be causing the problem.
I wish there was a simple and reliable way to test for this sort of thing, but if there is I'm not aware of it.
Jerry