If someone made an anonymous call to be vindictive and she is truly clean, she should let these child protection service people know that she is ready and willing, forthwith!
The child protection people have the duty of looking into any allegations that if true, have the potential to adversely effect the child(ren). While I don't know the exact figures, I imagine that in the majority of cases, where people making calls anonymously to report endangerment, physical abuse, drug use, etc. on a part of the parent(s) it has proven to be a legitimate claim on a variety of levels.
When they are investigating a claim that a parent is using, there is only one proven method to determine if the parent is in fact using or not, test them! Otherwise the claims of she's clean, are un-proven. You don't know if your friend is using unless you are able to and have tested her yourself, so stop being a codependent and tell her to take the damn test! Your opinion is not proof, period!
No one can be 100% sure that what they see tells the story. In a case with drugs, they have to have lab work to substantiate weather a claim is true or false, there's no other way.
Put yourself in the case workers shoes. You have seen time and time again, that in the majority of cases where parent(s) are accused of drug addiction and they are not users, they are more than happy to put the claim to rest by submitting to a test on the spot.
You also see that in the majority of cases where the parent(s) are indeed guilty of doing illegal drugs, they begin to look for ways to delay the testing or put it off altogether by claiming some right is being violated. Just like your friend is in the process of doing right now. What would you think if you were in their shoes? The same thing you should be thinking now, why? why not test? why not prove what your so sure of? why, why, why???
Until she does, she is just postponing the inevitable. Because, if the anonymous party reporting this allegation, did so to be vindictive, knowing the claim is false and your friend doesn't cooperate with the requests to medically prove it, one way or the other, she will be looked at as if she's guilty. Then, the anonymous caller can, at will, make another anonymous call whenever she wants and begin this cycle all over again and again and again. So until your friend proves otherwise, they are going to act as if there is some merit to these supposedly false claims.
Remember, they are only interested in the well being of the child If your friend is clean and she anxiously agrees to the test being done immediately, then comes up clean, she can nip this thing in the bud. Chances are, she would already have ended the whole mess.
She will also have ended it while proving her credibility in the eyes of the child services folks and future allegations of this sort will be taken with a grain of salt.
However, the longer she delays, the more it will appear as if she was buying time to "clean up".
Think about it this way, if your friend doesn't have anything to hide, all she has to do is piss! We all do it, several times a day. She's being asked to do something she will be doing anyway. So it's a bottle instead of a bowl, tell her squat on down and put a stop to this crap now.
If for any reason, she keeps putting it off or refuses altogether, your friend is on dope, it's as simple as that......