Husband co parenting with his bm

We recently put our children in therapy along with us starting marriage couseling. My husbands sons therapist would like to do a family session with him and his childs mother. My husband doesnt really want to do a family session. We share 50/50 custody with her pick up drop off set time and meet at the police station. She has always been a problem in our relatuonship and she is very ignorant and childish so he rather just not communicate. We downloaded a coparenting app so she is aware of all the appt and have school, doctors info. Should he do the family session? Do you think its appropriate for me to be there? We initiated therapy for our kids and are doing it as a family.