There are a few issues involved in this question:

1. To answer this, we must determine what the provisions are of the divorce decree. This document spells out parents’ rights. In joint custody, both parents have equal rights to information about treatment of their child. The divorce decree may spell out who must be notified or give consent for any medical treatment of the child. Counseling falls in this category.
2. In general, treating a child without the knowledge of the ex puts the child in the uncomfortable position of having to keep a secret from one of their parents. This burden is often counter-productive to the child’s welfare.
3. Often this question is asked in high-conflict divorce situations. These types of divorces include harmful stressors for children. The counselor may first need to address the continued "high-conflict" condition, starting with one willing parent. Counseling can help a parent find ways of reducing the level of conflict as a way of protecting the child from further harm. Once emotional safety for the child is established and the child is out of the middle of the conflict, counseling can begin for the child.

In some cases, courts may have given full custody to one parent, or limited the rights of the non-custodial parent. In these cases it may be acceptable to see the child without the other parent knowing or being entitled to any information.