However in this case the article states that initial request was to "strip search" the girl so the stakes started high.
The 20/20 investigation I just watched makes it clear that's not quite true.
He starts out slow - one piece of clothing at a time. This gradual scale is like boiling a lobster: it fools people the same way. After 2 hours of imprisonment, doing idiot things like jumping jacks, and getting spanked for failure to obey, that girl would have done anything. No blame on her - except for the initial failure to understand her own rights & limits.
As Roadtoad said, "dumb kid" is redundant.
The manager is not mentally retarded; she is morally defective. She saw a girl crying - her own employee - and simply didn't care about that as much as she cared about looking good to the authorities. That's a moral problem, not a mental one.
I would like to point out that the janitor is the one who stopped it all, simply by asserting "Something is wrong." Once he required the manager to actually check facts, the whole scam fell apart.
It only took one skeptic to ask a question. Nobody else ever thought to question the premises. Much like people grow up believing that 90% of the planet will go to eternal torment and yet God is loving. They simply don't question the premises.
The more I think about this the person with the most responsibility is Summers,
Heck ya. I can even understand Nix; I mean, he was acting rationally. Spank a naked 18 year old and get a blow job, free of charge? Where's the downside in that?

(the latest info says he got off with probation, which I find almost as disturbing as the orginal incident. This SOB should be doing 20 years.)
But Summers put the most trivial of her own concerns (looking good with her invisible bosses) above terrible injury to someone under her authority. This woman belongs in prison. It is the only environment our society offers that is appropriate for people like her.