'Police' is a tricky concept in China. When the Uni wants students to direct people towards somewhere, it will give them something that says Police on it.
You have the Peoples armed Police, and they're actually the only ones that i really regard as Police.
Then you have the Chengguan. They're more like inspection officials. Sure, they have powers, like all officials do, but maybe they might not have jurisdiction inside the mall.
I honestly don't know. I sort of think that if they have the gold, blue and red badge on their sleeve, they're real Police. Chenguan all have local badges. I've never had trouble telling them apart. I'd be surprised if Chenguan got mixed up in dealing with foreigners unless they had to. In fact i've always assumed they've been told to keep the hell away from foreigners if they can.
Is there some colour thing with the uniforms as well. Chenguan tend to be in vlue, even if a very dark clue, whereas the real police have that olive green thing going on.
Anyway, as stil says, the cameras in the mall were probably what was putting them off. If the girl really thought that she had a case, she'd probably have gone to the real police. With the cameras in the mall, the Police would have had no real problem tracking down your nephew. I'm sure the Police are quite happy to deal with foreigners who really do misbehave, but I'm equally sure they're wary of people making things up to try to extort money, which obviously does happen.
If the alledged harrassment didn't take place in a mall or a place with witnesss or cameras, then there's a good chance that this will go no further.