Also, as a warning to others, my friend was not trying to dilute. He thinks that maybe he just drinks a lot more water than other people do, all of the time. That's why it's good to home test before the real test. This is another reason why these tests shouldn't be used in the workplace. If people drank as much water as the NIH recommends we'd all test positive for over dilluted.
If your firend wants to continue to enjoy the benficial qualities of cannabis I still recommend he explore his options of subbing with synthetic urine. Its a pain in the ass to be held as a slave to your employer because of some false idea that Cannabis is bad. Its also why drug testing in the workplace should be illegal unless there ie reasonable suspiscion that he's under the effects of drugs at work. Any other drug testing is unconstitutional and violates both the 4th and 5th Amendment.