Generally, the police will not get involved in divorce matters. I would recommend discussing the situation with your attorney to see if filing a motion for reconsideration is appropriate.
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I believe your friend's parents are misreading their divorce decree. The age of majority in the state of Michigan is 18. Once a child reaches the age of 18 they have the legal right to choose where they live. However, in the situation of a divorce, the obligation of a parent to pay child support does continue until the child graduates from high school or 19 1/2 whichever is later, so long as the child is continuing in school full time.
While it is certainly upsetting to have someone intrude on your parenting time by reporting back to your ex-wife, I do not believe this continues grounds for a Personal Protection Order, commonly known as a harassment restraining order. I would recommend speaking with an attorney to see if there are additional facts which may warrant grounds for the filing of a Personal Protection Order.