It sounded like he chambers the first shot from the mag himself, fires it, and number 2 hangs up. Again, downloading the mags cures the problem for him.
He's just wondering if he wasted money on the McCormick mag, or if this is normal "new gun" teething.
Are you physically close enough to him to let him try your mags in his pistol, and/or for you to try his mags in yours?
Alternately, the fact that a down-loaded magazine works sounds like the magazine spring tension may be a bit light. I think I'd try a GI or Chip McCormick magazine, especially if his mags are 8-round capacity mags.
The Brits consider the magazines for a Browning GP to be 12 round capacity, though I can get 14 in mine with no difficulties. But for business, I do as the old Sar-major would have said and load a dozen, with the lucky 13th up the spout. Sten guns worked better with 28 or so loaded in some magazines rather than the full 32, as well.
His mags are 7-rounders that came with the gun, and one 8-shot McCormick PowerMag. I always thought that a light mag spring would jam the LAST round, not the first(second). Seems the pressure on the first couple of rounds would always be the strongest.