Depending on certain factors, especially if he was by any chance <18 when the crime occurred, he can get the record suppressed. Even if he was of age, there are certain processes to expunge records. It would behoove him to find out if he has any legal options in that regard.
Otherwise, Idio is right: short of lying, volunteer no information!
And know what the record says, so that if an interviewer slyly says "tell me about your legal troubles," he doesn't refer to an as-yet-unknown offense. Also bad would be to respond "to which legal trouble are you referring?," inherently suggesting multiple arrests.
The only thing is, there might be some disparity in what various background checks reveal, depending on the source. So the best bet might be to discuss his most recent speeding ticket, and let the interviewer up the ante.
And above all else, tell him to act humble (but not ashamed), and not to use the ever-so-subtly bragging tone he employs to impress the floozies down at the corner bar with his bad-boy arrest history.