Know that most SAT questions, especially in math, aren't really testing your ability to use known methods, they are testing your ability to quickly find the correct answer, by any means necessary. Usually you can wipe out two answers immediately, so then, if you don't know the process, or worse, know it and know it'll just take too long, resort to plug and chug.
Also, a wrong answer is worse than no answer. Wrong answers are subtracted from your score, no answer has zero input.
Don't ever treat it like some "all-important test". You can retake it if need be, and adding pressure is senseless and will work against you. Get a good sleep, have a badass breakfast, and then try to actively have FUN with the test.

Oh, and make sure you take a massive poo before. Being forced to sit there for an hour when you have to shit is going to ruin everything.

That'd be my advise for them.

advice*

Unless your friend is trying to get into an ivy league or get get scholarships based on sat, they don't need to worry about scoring crazy high. Worst comes to worst, they can take it, see what the questions are like, study for it a second time and then take it again.