Does this need to be "fixed" now? Are they applying for early college or anything like that? Barring LDs or anything like that, I would expect that reading high-level books and continuing to develop along their current trajectories will take care of this issue. If their general writing scores are already college-ready, they're obviously doing pretty well in this general area.
You certainly can get reliant on spellcheck, but I doubt that's the issue (there's no spelling on the SAT, surely?) MS Word's grammar checker is a pretty useless thing, so relying on that probably isn't the problem either.
I wouldn't waste my time on learning technical grammar rules, personally. If I wanted to improve mechanics, I'd probably give my child some writing containing errors to rewrite/"correct." My second-grader gets worksheets like these, so I know they exist. I think this is a nice organic way to get an idea of what a student knows as far as mechanics. You may need to back this up with some light grammar study, but possibly not. Many students will be able to learn the patterns without a lot of rote rule-learning.
