There is a “radical” concept now taught in many expensive MBA programs (Ivy League, etc.) that a small percentage of customers (let’s say 1%) account for an enormous and absolute percentage of a business total customer service costs (let’s say 90% of all customer service costs.) These customers have a “negative value” and should be fired, both for direct bottom-line reasons and as part of an overall good customer service program.
This 1% of bad customers costing you 90% percent of your customer service budget is a rough generalization, but reasonable for picturing the general scope; the actual numbers tend to vary considerably across industries, although they are usually similar for most businesses within a given industry.
The real point being, there is an effort in rarefied business circles to identify those customers that are really nothing but drains on the business (and that includes emotional and time drains on the employees, aka “human capital” as captains of industry are so fondly wont to call their people), and fire them. Not just refuse to humour nor merely discourage; outright get rid of them as customers.
Of course, anyone who has actually worked in customer service knows that the customer is not always right. 99% of the time, certainly. And if there is any reasonable question or grey area, it usually remains best policy to give the customer the benefit of the doubt. But that deliberately dishonest and loathsome 1%; especially when they so blatantly self-identify; sound business practice would show them the door, politely, but firmly. None but that loathsome pair derived benefit from the eventual managerial caving; not the business’ profit, not the employees, not the unhappily inconvenienced good customers. I would expect nothing but further grief and damage to the bottom line to result from any further interaction with those people.
It is an excellent general business policy, to immediately call security personnel or the police if an employee ever has even slight concern for her personal safety. Bright and competent people should be encouraged to listen to their gut feelings, and act on them right away. There is absolutely no reason to place fear of a false alarm over risking one’s personal safety. There is the additional benefit that when confronted by the knowledge that the police are en-route, loathsome customers often promptly disappear, never to be heard from again. Con artists frequently have records and current warrants out.
In short, the manager made a mistake in deciding to take this order; for purely business reasons as well as the obvious human and ethical reasons. I’m sure he thought he was doing right by adhering to the (outmoded) “the customer is right 100% of the time” rule, when it actually should be “the customer is right 99.9% of the time, but show that other .1% the door.” As previously suggested, how many good customers that happened to be in the store won’t be back again, because they disliked that loathsome pair as much as you did, and know those two nasties will be returning? Firing the outrageously bad customers is a key component of overall good customer service. It is an extreme step, and not to be done lightly, but in this case, it was clearly warranted.
The DM would do the business a service by suggesting the managers consider these points. Or a consultant could be engaged to recommend it. This would probably only cost a few thousand or so . . . 
