Different bedtimes happen. Undoing every routine the night after an exchange is something else. Kids feel the difference even when adults call it "freedom" or "just how we do things here."
Counter-parenting shows up as opposition for opposition's sake: reversing agreements, competing with gifts, blocking reasonable activities, bad-mouthing. Not every disagreement qualifies. Look for the pattern.
Your strongest move is boring consistency in your house. Keep your routines. Keep your values. If they buy a louder toy, you do not need a louder toy. You need to be the adult who does not enter the race.
Talk about your house rules without attacking the other home. "Screens off at 8 here." Kids can handle different homes. They struggle when difference becomes war.
Limit adult talk to logistics. Confirm plans in writing. Choose a few non-negotiables tied to health, school, and safety. Let smaller frictions die of neglect.
If schooling, sleep, or emotional safety is taking a hit, bring facts to a qualified counselor or attorney. Where intimidation or abuse is involved, ask for safety guidance before considering a joint process. Skip the speech. Structure will not cure spite, but it shrinks the openings spite uses.
Stay on your child's side. That usually means staying out of the contest.