If your daughter's probation officer has requested that she come in to his office today, then she should absolutely go in and meet with him. If she doesn't, it will only cause her more problems, and can work against her when she goes back in to court before the judge.
From the information you shared, I understand there's some discrepancy here-- the counselor says your daughter failed a drug test, when your daughter says one was not even given to her.
To answer your question-- she may or may not be given a chance to explain her side of things to the probation officer, that will be up to the discretion of her particular probation officer, how strict or lenient he is, and whether he does in fact have a positive drug test in his possession.
Based on what the counselor is alleging though, and the fact that the probation officer has asked her to come in, it sounds like your daughter is being accused of violating her probation, and she is most likely going to be taken into custody when she goes to the probation officer's office.
I'll let you know, that even if the probation officer won't hear your daughter out, the place where she needs representation to tell her side of the story and where it will really matter, is at her probation violation hearing. Let me explain a little about the applicable legal process-- even though your daughter may end up being arrested for a probation violation today, they still have to prove that a violation actually occurred. This takes place at the probation violation hearing (pvh).
At the pvh, the prosecutor will present evidence against your daughter, for example the results of the failed drug test (if in fact there was a drug test administered, I know this point is in dispute), testimony from the counselor, and maybe testimony from the probation officer. Your daughter will want/need an attorney to represent her at the pvh; these types of hearing tend to be very favorable for the prosecution, as the standards of proof are lower than at a regular trial, so that's why it's extremely important that your daughter have legal counsel that can represent her at the hearing, if in fact she ends up being arrested for a violation.
Potential penalties if she's found guilty include extension of her probation, possible jail time, more rehabilitation classes, and is in the judge's discretion after hearing all of the evidence and testimony. The penalties can be severe, so again, it's important that she have an attorney to represent her.
I hope that helps! Please let me know if you have any other questions. Also, please take a moment to rate me using the stars at the top of the page, as it's the only way experts are compensated for our time on the site. I do not receive a salary here at JustAnswer, and am only compensated for my time after receiving a rating of 3 stars or more. It doesn't cost you anything to rate me, and we can continue our conversation for free after. Thanks, ***** ***** it!