I really couldn't tell him whether it was a good idea or not.
NOT.
He had mentioned...38/357 then .32 s&w, .32 long, .32 H&R, .327 then he went into .45acp, .45gap, and something else that I don't remember with the point being that "lots of things were interchangeable."
It is really never a good idea to shoot anything in an
autopistol other than the specific round that the pistol is chambered for. The exceptions are rounds that are identically or essentially identically dimensioned (i.e. .30Mauser/.30Tokarev, .38ACP/.38Super,
assuming that the pistol can tolerate the pressure difference if there is a difference.
Otherwise, shooting anything other than the native chambering requires, at a minimum, a conversion barrel for safety.
In a revolver, things are different, and as long as the round will chamber
AND it is the same pressure or lower as the native chambering, it should be safe to fire. All of the chamberings and "interchangeable" rounds he mentions are primarily revolver cartridges. The .45ACP and .45GAP can be interchangeable, but only in a revolver, and only in a revolver that is rated for .45ACP+P.
There are three reasons that it is a bad idea to mix chamberings in an autopistol.
1. The round headspaces off the extractor which stresses the extractor since the extractor is not designed for that purpose.
2. The round headspaces off the extractor which can result in excessive headspace since the extractor is not designed to control headspace.
3. The sharp lip at the front of an autopistol chamber can shave material and retain it. If enough material builds up on the lip, it can act as a partial obstruction and blow the chamber when the gun fires. In a revolver, the chamber is tapered and there is no sharp lip to shave and retain material.
In the particular case under discussion, the bore size mismatch makes it an even worse idea since the larger bullet is bound to shave material at the front of the chamber.
In addition to stopping his practice, he needs to THOROUGHLY clean the barrel, taking special care to make sure that the chamber is completely free of any buildup before he shoots the gun again.