This is one of the hardest things about being a parent.. Allowing your child to fail. SHE needs to understand SHE is responsible for her own grades.
If she has to retake the class, so be it, it is her problem. There is summer school, sometimes online classes.. Or she can retake it next school year.. Quit trying to save her. Soon she will be in college or working and you will not be there to save her.
Does she do the homework, but just not turn it in?
My husband had this problem in high school.. he is ADHD.. IF he did the homework, then he would forget to turn it in.. Or just would not turn it in.. Drove his parents CRAZY!
SHE needs to figure out a way to keep up with homework, do the work and turn it in.. She needs to find that solution.. otherwise this is going to continue..
If she turns it in late, sometimes, teacher will give a late grade, but it is really not fair for teachers to have to go back and grade old papers.