The traditional case would involve the delivering carrier receiving a reasonably hefty fine from the refusing country and then flying the pax back to their point of origin (most times this happens the pax is legally resident in the origin port). There are a number of reasons why that isn't so straightforward here (or in a number of other cases), which is why I think the actual practical truth is going to be whatever the CBP officer on duty decides, and here the "ideal bureaucrat theorem" applies where the bureaucrat (CBP officer) who is paid to do a job but isn't morally invested in it will choose whatever path ultimately leads to the smallest amount of paperwork or hassle for them (maybe getting someone to watch her airside without processing a US entry and then loading her onto the onward flight -- maybe holding her for a while until UA can send her back to either YAP or the mainland).

TIMATIC is usually good for catching this at departure because the delivering carrier really, really does not want to be involved in a situation like this. I'm not terribly sure what the obligation is for a carrier who takes a citizen of C from A (exiting from a single-entry now-void visa) to B (where they are inadmissible), but I would assume they have to ultimately transport them back to C at their expense. It gets even worse in the more difficult case of someone who is not legally allowed entry anywhere (maybe dump them at LYR ) but I think that's pretty rare.