Dear T.:
Her parents can really just do one thing, if hiring outside help is not an option. They'll have to do it themselves. I went through it with my son in 1st grade - he was hopelessly behind in reading it seemed.
1) Cancel all TV and other 'fun' activities except for a short play time with friends and maybe something special on a weekend day. Not as punishment, but as a time saver. Make sure all TVs and radios are off when you start. Make it clear to her that she needs to catch up in order not to repeat, so she does not feel punished.
2) Go through the whole program with her yourself. If it is anything like our school, they may have weekly spelling and/or vocabulary tests. Do one a day to catch up, and don't forget the current one (to keep up in the present). Redo the homeworks and watch where she struggles. Find out whether she does not understand the question or the topic. I found that sometimes my son did not understand the question, and when I rephrased or explained it, he knew the answer.
3) Read to her and have her read. Reading visualizes spelling. Make sure she pauses correctly and reads what is there. If she reads 'father' instead of 'farther', it may not be the pronunciation but sloppy reading where she missed the 'r'. Also, if you have any sign of recurring slurs or omissions, talk to the school about speech therapy. Have her read a lot below grade level (for confidence and speed), a normal portion at grade level (to remain at par) and a little above grade level (for the challenge).
4) In math you have to take a two-step approach. First make sure she knows how to do something. E.g., if you need to subtract with regrouping, she needs to fully understand what to do. 9 4 1 - 3 1 5 becomes 9 3 11 - 3 1 5, leading to 6 2 6 of course. In the second step, she needs to practise this many times to let it sink in. Doing step 2 without step 1 will not yield results. I also found it helpful to give my son an occasional 8- or 9-digit number set instead of 3 or 4 to make him lose the fear of large numbers.
When her grades go up, reward her with an occasional TV show or an extra movie visit to excite her. Make it clear that it is for her progress, and that there is more where that came from. This also shows her that eventually she won't have to work so hard. Tell her that once she is at par with all 'B's and better, she won't have to do extra work.
Finally, cooperate with the teachers and ask them to reward her for 'C's and 'D's, given where she is coming from. Have them give her praise for what others do not consider so great a grade.
When I did that, my son was at half the speed he needed in November (2004) and the teachers were about to recommend a repetition of first grade. They did not think he'd make it. I found out EXACTLY in which areas he needed the most help, which happened to be reading and some math. In January, he was at 5/8 of the (upward moving) target, the tests where in April. Since learning curves get steeper later, he was able to catch up and make it. But I let my son know what was at stake, and I did read with him excessively. I also filled him in on his status. The teachers were nice enough to schedule his final test at the very end to give him more time.
I hope this helps a bit, but there's no shortcut to her work efforts. Unfortunately, it also requires a lot of her parents' time (or possibly yours as well?).
Regards,
W.