Unethical Therapy Practices

This sounds like a potentially serious situation. The most important thing is to make sure the child is safe.  If you suspect that a child is (or has been) harmed, I would strongly recommend that you report your concerns to your local law enforcement or to Child Protective Services. If you don't think that a child is in danger, you can still report your suspicion of counselor misconduct to the state regulatory board.

I will admit I am confused about this question. Are you the other parent in question or a concerned coworker of this therapist? 

Therapists do not prescribe or have access to medication. Most therapists won't even get involved in a custody battle. They know there is a good chance of their records ending up in court and that they themselves will have to testify in court. Of course, therapists are human and can make mistakes, but I find it highly unlikely that a therapist would knowingly and willingly do something like this that is such a major ethical violation. Not only are they losing their license to practice by doing this, they are going to face criminal charges and jail time. If their clinical judgment is that the other parent is a danger to the child, they don't have to resort to illegal means to try to prove this. 

If you have concrete proof that this has happened then there is a process in which you can file a report against the therapist with their licensure board. Every therapist is licensed in their state. There are different types of licensures so you would need to know this therapist's licensure.

Most accusations from the other parent are expected in a custody situation because people do tend to get quite ugly. Don't accuse the therapist or the parent of anything just to try to make yourself look better and them look bad. Have evidence of wrongdoing before accusing them of this. If this has really happened then of course it needs to be reported, but don't try to ruin someone's career unless you are sure this occurred.