my suggestion is if the other parent of the child is emotionally abusing the child by bad mouthing and making threats to the other parent (you) in front of / to the child, which would cause a child to do the same its time to make some parenting changes like modifying the visitation schedule to less and supervised visitation. and possibly a report to child protective services and the family courts law guardian of the child, if there is one. maybe even a protection order protecting this childs mental health and therapy for you and your child to help get this child more stable. a child learns from their parents. if a child learns how to domestic abuse someone including bad mouthing the other parent then they run a higher risk of being domestic violence abusers as an adult.

my opinion its time to not let the child get away with it and take steps to ensure this child is not going to grow up thinking its ok to do this with mom and dad so its ok to do with their more adult relationships and possibly end up domestically violating their own wife and children in the future.