I bounced this off someone I know who's a partner in an ownership group with about half a dozen dealerships, and worked way up via every job there is in the dealership from grease monkey and salesman to service manager, sales manager, and GM. Here's a summary of what he said:
Why would the GM do this? Only reasons he can think of is that either the GM is a moron, or the deal was made for such a low price they couldn't afford to deliver at that price, and the GM needed some excuse to cancel it.
Customer's situation: First, does the customer have a signed sales agreement with a down payment made? If so, he can probably sue for specific performance and get the car delivered at the agreed price, although his legal costs may exceed the cost of going to the Infiniti dealer in the next town and buying the car there. Since the car he ordered is now sitting in the first dealer's lot, he can probably get a "dealer exchange" to get the actual car he originally ordered delivered at the second dealership. Second, given the presence of a third party, the customer could sue for the humiliation for the scene in the salesman's office, although success isn't likely. All in all, his best bet is to just get the car elsewhere, and if he's in a vindictive mood, to pass the word around the gay community to buy their cars elsewhere and complain to Nissan/Infiniti regional/national.
Salesman's situation: He might have a case for wrongful discharge, but it will depend on just what it says in the dealer's personnel/policy manuals. He can take this to a lawyer in that state for a read on that, but why would he want this job back, anyway? If he's a good car salesman, he'll have no problem getting a job elsewhere, and if the GM says anything negative about the salesman to other prospective employers, that could be actionable.
As for taking it to Nissan/Infiniti, if they hear about this, it's unlikely they'll yank the franchise just on this story alone, but the owner might get some nasty mail from regional/national management. The fact that this is a franchise, not a company-owned store, reduces corporate's leverage.
That said, there was a case in 1996 where where the owner of a Nissan dealership lost his franchise over the use of the word "******" in reference to a black employee -- on tape, which ended up on local TV. Google "Nissan Crumpler ******" for plenty of references, including a Nissan spokesman's statement that the owner "has a history of behavior there that's below common decency." If the local gay community runs this story past the local media, and the GM is dumb enough to let them interview him about it and confirm what he said, and the GM has a history of such behavior, this situation could get much worse for the dealership.