Ordered to Pay Arrears Years After Children Reached Adulthood

My question involves child support in the State of: Michigan.

I am asking on behalf of my girlfriend. She just received a letter from the courts saying she owes 30,000 in arrears for her kids. At his time the kids are 28 and 25. During the time that they were growing up they split the time living between both parents, but nothing was put threw the courts of where the kids were living and no support was paid on an agreement between the parents. Now the court is saying she owes this money. If her EX states that she is not obligated to pay him and is willing to put this in front of the court can the arrears be forgiven?

Were they ever on public aid?