I'll be nice and not say he's crazy, but I think he is.
He may be a smart guy, but he sounds irresponsible to me, even if he doesn't realize it. We "own" every projectile we fire, and a shotshell being fired while getting into firing position sends a lot of projectiles flying about that we have little control of.
We have no idea what the situation may be if we need to use our gun.
What if he needs to shoot someone standing near his child or grandchild? Snapping a shot round in the general direction as he lines up the gun is far from the best course of action there.
I suppose in that situation he could advance the revolver cylinder one round or rack the auto's slide to bypass the shot round...but I don't think the bad guys like to wait around for us to get ready.
Besides, there's also the argument that the shotshell is doing little beyond delaying the delivery of properly placed bullets that will do damage. I wouldn't think a sloppy hit by a few shot pellets would have much effect on a determined attacker, and would only delay the deliver of a decent bullet by a fraction of a second.
If you can get your hands on a timer, take him to the range and have him demonstrate this technique.
Then have him practice drawing and firing the usual way, and time him then.
I bet that he gets that wild shotshell off only a fraction of a second faster (maybe as little as .1 second) than his first shot the usual way...but I also bet that the firing of the shotshell delays the firing of a "real" bullet by more than he gained- maybe .3 second or more.
In other words, he is getting a wildly aimed, relatively ineffective, shotshell round fired ever so slightly quicker than by taking a good shot with a real bullet, but it delays the firing of his second (better placed and more effective) shot- the one that will do some good.
You say he's "trained with all the pros". I have a feeling it wasn't one of them who gave him his idea. If so, I'd like to know which one, so I know who to avoid.
Many people have been exposed to training. A smaller number have been trained.
But no, I'll be nice and won't say he's crazy.