John, your right, I don't "absolutely" know that for sure. You're correct, I am making an assumption before physically examining the lens in question myself. And yes, the other lens his buyer has, the 50mm 'Lux, could have alignment problems as well. The body cam could be out of adjustment just as easily too. Or a combination of all three possibly. But that frankly is not likely, given that both lenses are not off, just the 28mm. Having owned both, I dismiss the body and user error, as the 50mm 'Lux is much more sensitive to calibration faults than the 28 'Cron is.
It is also certainly possible that the lens took a severe shock in shipping serious enough to knock the front element loose, but that also is going to take a professional examination to determine. Not something your going to fix in your garage shop in either case. What some people forget is though a technician may be Leica trained and certified (I wasn't aware Leica certified outside technicians either, when did they start doing that?) they do not always have access to the very sensitive & extremely expensive equipment to properly calibrate to the standards that Leica does. 0.01 is a very tight tolerance.
Kindermann up in Canada used to be the Leica distributor for Canada, but gave it up as the gear necessary to do repairs was way too costly to afford for such a relatively small market. They had Leica trained and certified warranty repair technicians on staff, and back when they did repairs were a lot faster than Leica in the States or in Germany on repair turnarounds. But they do not own the test gigs necessary for alignment or repair of the newer lenses, which the 28 'Cron falls under the description of.
Given that nobody but Leica themselves (who actually MAKE their own tools!) own precise enough tools to do the alignment perfectly (call Leica NJ and ask them, or call Kindrmann directly to check for yourself), and given that I heard nothing from anybody about the lens being bad before it went in for a simple CLA, combined with the 50mm 'Lux working perfectly (that lens is a real bear when it's off), I will stand by my first conclusion, the lens is out and needs to go to someone who can properly align it correctly. Which, to my understanding, is either to Leica in NJ (who now has invested the huge cost in the necessary alignment gear, and can repair most everything M save for the Noct I'm told) or directly to Solms in Germany. I wouldn't trust a problem lens to anyone else, as nobody else has the correct equipment to do that fine an adjustment to guarantee the result. This is an exacting procedure, NOT for the faint of heart.
It's so hard to do right, that even Leica sometimes blows it, and the lens needs to go back for correction.
As always, just my opinion.