If there are orders in place detailing how custody (legal decision making) and child support are to be followed your friend should start by reading the orders. If there has been a change in circumstances relating to child support, parenting time, he may wish to file a petition to modify the order. If she is not following the orders he could file a petition to enforce the orders. You can find forms on the webpage to the Maricopa County Superior Court (check the self-service area).
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What your friend needs to do is request a modification of the current custody order and parenting plan. If your friend's ex has been denying him access to the child and not following the current parenting plan, that will go in his favor when the judge decides the new custody arrangement. You can request a recalculation of child support when you modify the parenting plan, or request a modification by itself if you think there is an error in the current calculation.
This post is general information about divorce and family law in Arizona, and is not legal advice. Divorce is a complex process, and taking legal action without the one-on-one professional advice of a lawyer can produce unexpected results.
It's impossible to give statistics. Every case is based on its own facts and ultimately what is in the best interest of the children. Both of the attorneys who answered have given you excellent guidance on how to proceed.
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I agree with the answers above, however, I would add that there is always a timing issue when it comes to filing Petitions to Modify. In the event the ruling within 1 year, then he may not file a Petition to Modify, unless: 1) You can show the child is in serious danger. If you have enough evidence to show that Mother is neglecting the child's medical needs with respect to diabetes, then that may suffice to allow you to file the Petition. 2) You can show the other party has violated the Decree. If the other party has violated the Decree, a Petition may be filed 6 months after the modification has been entered.
Regardless of when you file, you need to show the court that there has been a continuing and significant change in circumstances, and that any modification requested is in the best interest of the minor child. There also may be an Order indicating that mediation be attempted prior to filing. As such, you may have to explore mediation first, unless he believes the child is in serious danger.
This answer is provided for general information only and is not to be construed as legal advice. This response is not intended to create an attorney-client relationship.