you are right and partially wrong.
In the contractual situation like you described, then yes JWs would not continue to pursue business or enter a new business partnership with someone who is DF/DA'd.
But in a situation where a JW works in a service industry and has no choice who comes into the door and requests service, whether that be waitressing, a shoe store, or a business office, in that event the JW may have "business" dealings with someone they know to be DF'd/DA'd
this latter situation describes the situation as Englishman originally put it. although it is not likely that the secretary was under any business obligation or job duty to provide tea to him, if she did it as a regular course of her conduct then she could have chosen to do so with him without (theocratic) consequence.
IN Englishman's latter post he provided details however that indicated that the Secretary was actually socializing while at work with him which only makes it stranger that she would then state that she couldn't provide him tea.
All of this nonsense only makes the point that when it comes to DF'ng in practice setting up all of these rules and regulations and fine lines, bright lines, gray lines, etc. is very much like the pharasaic tradition that few jewish persons were able to keep faithfully.