@ucbalumnus and others recommending community colleges for remedial math: if child and parent are on board to fix up math deficiencies, better to address it in high school with extra tutoring, and summer work. And it seems that OP has the resources to do so. Remedial work in college has not shown the type of gains that are widely assumed, but that is a topic for another thread.
I think it would be advisable to get a tutor with a lot of experience diagnosing deficiencies. As @juillet points out, hard work and encouragement should get the student to at least a reasonable level. The pace will depend on how much ground there is to make up. There are free diagnostic tests available on the Internet, in addition to software solutions I mentioned earlier. They can provide a road map to you and the tutor as to the nature of the topics that need to be mastered.