Quit School. No Longer Living in Mothers Home. Stolen Child Support.

[Hairy]

As is standard in Texas, my husband is required to pay child support until his daughter is 18 or graduates from HS, whichever occurs last.  While he was attempting to get a Graduation Verification Letter from the daughters school for Child Support reasons ( she turned 18 last December and was to graduate this June), it was discovered that she has not been enrolled in any school since February of 2007. Neither his daughter nor her mother ever mentioned it, go figure. It has also been discovered that the daughter has not been living in the mothers home, possibly also since February 2007.  Even though a job change in December lowered my husbands income to half of what he was making, we have continued to make the original payments based on his previous income, even though it has put us in some serious financial stress ( our choice, not the courts). Both the mother and the daughter have refused to respond to numerous emails, snail mails, and phone calls, initiated by both ourselves and the grandmother, since last October.
I'm so mad I could just spit.
Has anyone else experienced this? Are we entitled to get that money back? Is it worth taking back to court? Can I roast the #%^#@!!! over an open fire pit?
Any thoughts or help would be greatly appreciated.