I can think of several shotgun powders that look like little flat circles, but none that would be 15 gr in a 38Spl.
The two slowest powders I have used in 38 are 2400 (11gr) and 4227(14 gr). 4227 is a rifle powder and is extruded, not disks.
His comment "this is a faster burning powder," is very troubling. If a lot of powder is used, it should be a SLOW powder, not a faster one. He does not sound knowledgeable.
I suggest you send a 10 round sample of his ammo to NARCAN and make a complaint and explain your concerns.. Ask them to test the ammo.
Their pressure gun will take 100,000 psi. It might need it....
And ask the loader to be specific in telling you what powder he used. Maybe it is some kind of rifle powder.
Judging by the picture of the powder, it looks like a power I have. I have 40 pounds of powder that is the sweepings from the bench and floor of the Dillon that has loaded 500,000 rounds of 308 ammo. The powder was supposed to be disposed of, but it was kept in jugs marked "sweepings". It is a mixture of various ball and extruded powders running in the speed range from 3031 to RL15. I ran the powder through a window screen, to filter out the dead primers, bugs, nuts and bolts, etc and now use the powder for my own plinkers.
It looks to me that your loader buddy may be using his sweepings to load ammo he sells. It looks like a mixture of ball powder and flattened ball. Stop him, before someone gets hurt.