Is it safe?..I'll answer that question with, will the weapon explode from over pressure?..NO.
Would I do it?..again..NO..unless for some Godforsaken reason someone was gonna kill me and all I had was one 9mm Para bullet!
The bullet diameter of the .38 Super is exactly the same as the 9mm Luger, .355 to .356. 38 Super velocities typically run in the neighborhood of 9mm +P or +P+ as standard velcities. The 9mm has a shorter overall lenght than the .38 Super, so here starts the problems.
Your concerns should be two things.
1. Reliability. Headspacing is on the case mouth for most auto cartridges. Because of the free space, a shell could come loose from the extractor and lodge into the chamber.
2. Chamber erosion. If this practice is the norm, then the area where there is normally smooth chamber wall will become eroded. When a .38 super cartridge is then chambered and fired in such a chamber, then serious problems could occur.
I would suggest he purchase a 9mm barrel if and mags if he wants to shoot 9's and avoid future problems.
One last note...38 Auto ammo is safe to shoot in a .38 Super, but not vice versa.
Good SHooting
RED