They smoked weed about 3 weeks ago for the first time and about through up after one hit and they have a hair drug test next week will he pass?

Good: It's unlikely that one use will show up three weeks later. If it does, it will be at a low level and you can say it's secondhand smoke. This is not the place to argue the wisdom of the cannabis laws or screening for its use. I simply hope that under the present conditions, you'll obey the law, and also not use a more dangerous intoxicant simply because it clears the body faster.

If really ONE time..: ...Probably yes. Hair looks for longer term use over longer time. So may survive the test. Best to avoid drugs altogether.