Well, it depends. If the student has schools on the list that match up with their ‘mediocre’ grades then they should be able to apply TO.
OTOH, if they are trying to use a relatively higher test score to help them get into a more selective school than their grades alone would suggest, then you are right, they need a test score, and a strong one at that.
The question that strategy raises…why does a student with mediocre grades want to go to a more selective college than their three year academic record would suggest?
This might make sense for some students with a strong upward grade trend and/or with high course rigor, but probably not a good idea for many students with mediocre grades for three years.