"Was a wrong should i just have dealt with my own child?"
His parents may have a different understanding with their son than you with your daughter. It is always the best to deal with the issues that are closest to the heart and to the home and let others do the same. Your daughter most likely believes that she's an adult or legally almost one. She may resent this incident and consider it embarrassing. The truth is, she can still find other ways to do what she wants to do and become more skilled at deception. It would be more helpful to deal with her and in the privacy of your home hence forth. You want her to be able to trust you not resent your judgment. You may also view the incident as a mistake on her part being young and eager (very common to teens to act impulsively) She understands that her behavior may have not been appropriate (that is why she lied in the first place) Of course, you do not have to agree or disagree with her at this point. You may want to talk to her about what her choices may lead to later on and the things she'd have to face/cope with should let's say a pregnancy takes place or she gets in trouble legally(you've mentioned testing her for cannabis use)