Need to know how to pass a blood tezt

my friend was pulled over smoking a joint---they found no weed, but took his blood and made him sit in jail 6 hrs and charged him for driving under the influence if they can prove it,,,,he wants to have another blood test by a private party to refute the other test,,,,but would like to be sure to clean out before another tezt-----he just returned from Cancun and smoked some weed there---but the cops claimed they smelled weed when they approached the car and they took him away, searched with the dogs and found nothing