He may be one of the liucky ones. THC half life varies from 0.8 days to 10 days depending on the person. You info is vague as you say he smokes every weekend. How much, how often over the weekend, quality, his height and weight, his physical activity level, was it with the first void of the day?

It is possible for some people to get clean very quickly while others take forever. If the test was properly stored and in date, it should be very accurate nearly 97-99%.

You smoked once in June then on August 12th. It is safe to assume your body was completely clean before smoking on Aug 12th. Most people clear a one time high in 5 days. If you want go with a mild dilution, half the fluids all the aspirin and Tums.