I think there are times it is okay for parents to say negative things to their children about the other parent, and those times are
1. when the thing is true, AND 2. when the thing is a genuine safety issue.
"We left [parent] because parent was physically hurting you/physically hurting me, you need to avoid seeking out [parent], and if you see [parent] you need to run and find a grown up or a policeman and call me."
or "I am glad that you felt safe telling me that you are LGBTQ. I support you telling [other parent] if you want to, but you need to be warned that [other parent] will make you leave home and/or stop paying for your college fees if they know."
no subject
1. when the thing is true, AND
2. when the thing is a genuine safety issue.
"We left [parent] because parent was physically hurting you/physically hurting me, you need to avoid seeking out [parent], and if you see [parent] you need to run and find a grown up or a policeman and call me."
or "I am glad that you felt safe telling me that you are LGBTQ. I support you telling [other parent] if you want to, but you need to be warned that [other parent] will make you leave home and/or stop paying for your college fees if they know."
This is NOT one of those times.