If what your friend is saying is true, then he wouldve passed that same test that same day if he were to pour that detox potion down the drain and refilled the bottle with water, or any other fluid of his choosing.

ALL detox products "work" via the principle of dilution. Anytime you consume fluids (including the detox potions) you are diluting the THCA concentration that is present in the bladder. The product does not rid the body of THC, as the sales pitches would like the consumer to believe. They are a scam mainly because you can spend up to $200 on a magic elixir, and accomplish nothing different than what can be accomplished by using just water and vitamin B2.
Dilution techniques (whether its the potion, the N2 method, etc) are only temporary - once the fluids ingested had all been urinated out, the bladder is replenished with fresh THCA-laden urine by the kidneys and thus, the donor would resume testing positive for THC.