i dont know who told you that one use is detectable, or that products can be in some way detected.
As a rule of thumb, three uses is the threshold for detectability. Labs do not and cannot detect any cleansing product.
You ought to do some research about how tests are done, it'll make you feel more confident.
i would refer - your friend - to the marijuana.com forums. some people there have strong scientific credentials and have passed multiple tests despite heavy drug use.
Blood, Hair & Saliva Testing - Marijuana.com
look for posts from a user called 'Jerry G' and another user called 'Sec.' They're very experienced and will be able to advise you better than me.
But its my opinion that you don't need to worry about it.