I personally have been taught that even if someone instantly bombs the interview, you have to make them feel like they were given a decent shot so that they won't get upset and try to sue you.
Sadly you can't even give feedback on what people did wrong because anything that you say can and sometimes will be held against you in a court of law.
At the time, I was young developer being asked to interview a somebody. If I'm honest, I was a little prideful that I was interviewing him, and not one of the other, more mature developers, so I took him through the process.
I think, knowing what I know now, and feeling confident in who I am as a developer, I'd probably at least ask about it in the interview. I'd say "Why did you choose to wear that shirt today?" Serious question. I'd be interested. Maybe the answer is "Because I don't really want this job" and then it's clear. Maybe it's "Because I don't want to work at a place that gets offended easily" or maybe it's "Because I didn't have any other clean laundry." I cannot think of a valid reason to wear that shirt to an interview, but I'd at least be interested in his reasoning (though he's still not getting the job).
Thanks! I've contributed to an interview or two, but I haven't yet been tasked with the responsibility to cut one short if a serious red flag like that came up.
Sadly you can't even give feedback on what people did wrong because anything that you say can and sometimes will be held against you in a court of law.