SAT in general tests one's ability so not 补习 is needed. You see many 8th or 9th graders get very high SAT scores. Because they are very smart.
ACT generally test one's knowledge and how well you know the test format. So, 补习 ACT is more effective than 补习 SAT.
Since she is taking ACT the first time in Dec, she is at most a junior. There are plenty of time to improve the ACT score.
best of luck.