The negotiated protocol involves a re-test, if a masking agent is found in the urine. I suppose the timing could be just perfect, so that the diuretic happened to flush out the last detectable bit of PED, but the odds are that the re-test will uncover something. If he's taking a diuretic even after the PEDs are long gone, well, now we're into angels dancing on the head of a pin territory, and are back to the diuretics themselves being rightfully banned and therefore a stupid risk for no competitive benefit.