In Texas once the child reaches the age of 26 it’s over with. At least that’s how it used to be.
If this lady remarried and her and her husband claimed the kid as a dependent then I think the IRS should hit her for back taxes if she gets money from the old dude.
Child support is taxed before given to the custodial parent. I do not know how long claiming a stepchild (not claimed by another taxpayer has been allowed.