Other than that, I found his response reeking of condescension and inexperience.
I didn't catch anything in the post saying he didn't find that out until the interview.
It's like going out on a date with a girl you're not attracted to just for practice. And then when she asks why you didn't go on a second date saying, "Because you're frankly not very attractive." You were kind of a jerk for going on the date in the first place, but the feedback made you a certified one.
In any case either the candidate doesn't do the type of reasonable DD that I'd expect from an applicant, even a college hire. Or he's bordernline unethical.
You don't know that there's no chance of working there until you know what your other options are. If you're still interviewing (i.e. you don't have offers/rejections in hand yet), you don't know what options you have. You might know that you won't accept Company X's offer if Company Y also makes you an offer, but that's quite a different situation from knowing there's no way you'd accept Company X's offer.