That's interesting that creatinine levels were flagged as the reason for the sample registering as invalid but not specific gravity. Creatinine is much like urea in that it a nitrogenous waste product and marker of renal function. Presumably the value you quote for SWIY is 15.9mg/dl (i.e. 15.9mg/100ml urine), which is certainly on the low side and would strongly suggest sample dilution to testers.
Attached is a pretty useful document from Redwood Toxicology that explains the use of creatinine testing in relation to cannabis
testing, but also gives an overview of its general role in urinalysis. The most important part for SWIY is:
Originally Posted by Redwood Toxicology
Normal random urine creatinine concentrations range from 40-300 mg/dL in males and 37-250 mg/dL in females. Values as low as 17 mg/dL have been observed in rare instances. Certain physiological conditions such as diabetes may result in consistently dilute urine specimens.
SWIY's urine creatinine concentration was on the very low side. If SWIY was not diluting the sample or drinking excessive fluids to purge then they really should get their kidney function checked out by a doctor. Redwood note that medical conditions such as type II diabetes can affect urine creatinine levels, and presumably many other conditions that affect renal function can too.