custodial parent and back child support

My fiance owes his daughters mom back child support. She has told him many times that she would have it dropped but never has. She is now claiming that she can not just ask the court to have it dropped. Although his brothers girlfriend was able to have her children's fathers back child support forgiven in its entirety. Their child support agreement was in Hendricks County and that is why she states she can not do the same. If she truly does want to have his back support forgiven can the court grant this in Boone County, Indiana?