It sounds like she would benefit from doing math prep and may not need to do much for the English/Reading/EBRW for either test. If she can squeeze it in, it’s worth taking a timed test for each one - if one score is much better, then just focus on one test. If they are about the same, maybe she can focus on whichever one she likes. If the practice scores are high, maybe she won’t need to do much of anything in terms of prep.
My son took both tests in junior year (October SAT/December ACT). He didn’t do any prep other than a little Khan Academy math in the summer before, two timed SAT practice tests, and one ACT practice test. His practice scores suggested he would do about the same on both tests and he did (ACT was slightly higher). Actually, both of my kids did better on the ACT and neither one did any ACT specific prep.
Other than the science section, there is nothing on the ACT that wouldn’t be covered by SAT prep. With the ACT, you just need to implement the simple strategy of working as fast as possible (which is easier since the questions are more straightforward).