In my readings on conflict resolution I ran across a piece of advice from Hostage Negotiations.
The police negotiator will try to engage the constituents of the hostage taker in order to discuss the crisis with him.
The child in a custody situation could be viewed as a hostage when the other parent is hostile. So how does the police tactic apply?
Enlist the friends and family of the other parent, and show them how cooperative co-parenting ultimately benefits the child/ren. They may help to reduce the level of hostility.
The negotiator needs to find out why the crisis is being acted out. What are the roots of the hostility, and what is the true intention of the kidnapper?
Then it’s time to think about concessions… to cut a deal. I also found an interesting article here – https://www.psychologytoday.com/blog/promoting-empathy-your-teen/201306/three-rules-negotiating-child-custody