That certo trick doesn't work for everyone and when it does work it is usually because they drink a lot of fluids with it so in effect it is the dilution method. A 100% way to beat the test is to buy synthetic urine such as Quick Fix and keep it with you at all times. If they test you it can be ready in a few minutes to an hour. If you get your friend's clean urine it only lasts for 8-12 hours at room temp and you probably don't have a place at work to freeze it and then take it out and reheat without going unnoticed. Read the substitution thread on how to bring it and keep it to temp.