Actually, when I got interviewed for a PD reserved position they require you to list the firearms you own-with serial numbers. It's probably standard operating procedure for the background check.
I was a fed. Nobody at any time ever asked me diddly about guns--what I knew about them, how many I owned, had I ever shot, nothing.
'Course, they also had pulled what they could of my military jacket, so that answered more than a few questions.
I seriously doubt your friend was rejected simply because he owned too many guns. Guns are property, and property is constitutionally protected. LAPD and LASO have been sued WAY too many times over rights violations--they know better.
Personally? I think one of two things probably happened. One, his over all scores were below the beltline and there were better, more qualified candidates of the correct lineage and gender orientation ahead of him. This happens in DoJ agencies to a sickening degree, but try proving it.
Second, he just didn't dazzle the interview/selection board. They'll always tell you that you did "Fantastic!" or "Outstanding--best candidate we've had today!" but in reality, you're just getting jerked off for their own amusement.
Trust me, I've sat on a few--not by choice, but because my SAC was pissed at me. Helluva note, eh? Personnel becomes punishment.
Great line out of one of the Dirty Harry movies about Personnel. . . .
Still as true today as it was back then.
If your buddy really wants to be a cop, a big city agency isn't the way to go anymore--at least not for the majority of folks. I meet a lot of suburb cops out here in Texas that started with Dallas PD, LAPD, Chicago, Atlanta, etc--put in time, put in their applications and get the hell out as soon as they can.
Jeff