I have no idea about hair, but I think it can be revealed in a drug test for up to six weeks after use. He should be fine for a urine test if he hasn't smoked since Valentine's Day.
I just did a google search and found that hair follicle tests can find traces of drugs up to 90 days after the last use. Trace amount of drugs remain in the hair core and cannot be washed out. Hair grows at a rate of a half inch per month or 1-1/2 inch in 90 days. I'm not really sure how to interpret this information. Maybe he should buzz his hair super short.
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