If there is going to be a gap between test scores and grades, it is better to have the grades be better than the test scores. Colleges want to know who is going to perform in college. Performance in college requires the same skill set as high school performance. On the other hand, it is true that colleges screen on test scores too. I think that a trained test-specific tutor can be well worth the time and money investment. There are lots of options for test prep. For example, Kaplan offers one-on-one tutoring that can be costly but also offers group sessions (even some online). Depending on the student, group sessions can be useful too. Finally, some students get better scores on the ACT than the SAT. It is worth taking both tests at least once to see what happens.
More and more schools are offering the no-test option for applicants, even some high quality liberal arts colleges.
At the end of the day, the family needs to sit down and talk about goals, finances and target schools. It could be the case that the target schools are going to treat this sort of overall applicant package quite favorably.
Good luck!!!