Turning a rifle into a pistol can have some ramifications...
Better look into the laws on this. I seem to recall that there were legal issues with the old Broomhandle Mausers with the stock/holster, and of course, there’s minimum barrel length for any rifle or shotgun.
I don’t know what the legal ramifications are, but they might well get ugly.
Good luck!
Mark
If he keeps the barrel length to 16 inches or longer there is no problem, unless the feds have implemented a total over all length criteria since I last looked.