I have a friend taking the GMAT. Any words on advice if he's particularly shy about math?

The GMAT is adaptive now, and that can really throw people off if they aren’t prepared for it. Studying is always important, but I’d recommend also taking practice tests in the adaptive format, if possible.

definitely practice tests in the adaptive format, in my experience getting used to and comfortable with the format of a standardized test (including the main forms of questions) is at least as important as the content

For the quant section: www.bellcurves.com (sign up, $30/month)

Also, tell him that there are alot of gmat forums out there.
www.gmatclub.com
www.beatthegmat.com
www.testmagic.com

Take practice tests. Also, on at least 1–2 of the practice tests, do the writing section. Don’t worry to much about the score on the writing section (it isn’t figured in to your overall score), but writing for 1 hour and then jumping in to answer GMAT questions requires a bit of training.

Once you take a practice tests, look at the results. This will tell you what sections within Math you are weaker in. Spend time doing tons of problems on areas you are weaker in (e.g. problem solving, fractions, etc). Also, I highly recommend the Manhattan GMAT practice course. They provide great materials, the courses are taught by high quality instructors, and they give you 6 practice adaptive tests to take online. Remember, you won’t get better at the math section by avoiding it. Take a practice test, figure out where you are weak, and focus your time on improving in those areas. Good luck.