I would suggest getting a lawyer, if there's any way you can afford it, to deal with the counselor. Just the fact that she's willing to testify calls in to question her ethics.
Is she a LCSW,
psychologist, LPC, what? It's agains the APA's ethical guidelines for a psychologist to provide expert testimony regarding a client unless that relationship has been disclosed at the outset. A treating psychologist cannot be an unbiased witness for the court.
Each specialty has it's own ethical standards, but they're all relatively close. The fact that she is also treating the
BM and her other child is a clear conflict. She can treat them all together in family therapy, but not each one individually. See the following excerpt from the APA manual:
"3.05 Multiple Relationships
(a) A multiple relationship occurs when a psychologist is in a professional role with a person and (1) at the same time is in another role with the same person, (2) at the same time is in a relationship with a person closely associated with or related to the person with whom the psychologist has the professional relationship, or (3) promises to enter into another relationship in the future with the person or a person closely associated with or related to the person.
A psychologist refrains from entering into a multiple relationship if the multiple relationship could reasonably be expected to impair the psychologist's objectivity, competence, or effectiveness in performing his or her functions as a psychologist, or otherwise risks exploitation or harm to the person with whom the professional relationship exists.
Multiple relationships that would not reasonably be expected to cause impairment or risk exploitation or harm are not unethical.
(b) If a psychologist finds that, due to unforeseen factors, a potentially harmful multiple relationship has arisen, the psychologist takes reasonable steps to resolve it with due regard for the best interests of the affected person and maximal compliance with the Ethics Code.
(c) When psychologists are required by law, institutional policy, or extraordinary circumstances to serve in more than one role in judicial or administrative proceedings, at the outset they clarify role expectations and the extent of confidentiality and thereafter as changes occur. (See also Standards
3.04, Avoiding Harm, and
3.07, Third-Party Requests for Services.)"
You can find the whole manual here:
http://www.apa.org/ethics/code/index.aspx#
I would go after her on the basis of multiple relationships, and the fact that she can't be objective when it comes to the treatment of your child - especially in preserving the child's relationship with you (ie: causing no harm) when she is treating your ex on an individual basis.
File an ethics violation against that counselor and ask the court to stop your ex from taking the child there, or to anyone who refuses to also include you in therapy.