Has anyone ever had this problem ?

I'd tell her to find someone else to co-sign.

1) Pay the car note and deduct from the monthly child support check, including the receipt showing it has been paid. Hope that this is allowed by law or she does not push it

OR

2) Consult with the child support department at your local courthouse and see if option 1 is a legal option with no recourse.

OR

3) Bitch about it on an internet message board while she continues to ruin your credit.