How do hair drug tests work?

So my SO got a job interview for the railroad, he just randomly applied not expecting to get a job offer. He doesnt smoke weed and he doesn’t do any other drugs. Not at all. But over the weekend we took a trip to a cabin with friends and someone had a blunt, and we don’t smoke weed but we were all having a good time and just thought okay well why not, just gonna have a good time with friends. Well yesterday he was offered a job interview and it’s a super intense interview and they do a hair drug test. Before this weekend he never smoked weed, he took only a couple hits of the blunt and now were worried this one time will ruin a great job offer. Of course this would happen the ONE time he takes 1-2 hits of weed. Does anyone know if it would show up. This is just a reminder why we don’t do drugs in the first place 🙄 and we don’t.. just worried this could ruin a really good chance at a really good paying job.