Can a marriage counselor testify in a custody hearing?

I am asking on behalf of a friend who may have to go through a divorce and custody battle in the near future.
Is a marriage counselor legally allowed, without divulging the details of confidential discussions, to make recommendations about custody of the child/ren?

I would very much appreciate answers based on professional knowledge and/or personal experience, and I would also love links to, or suggestions of, online resources.

This is in New York.

Thank you.