If he doesn't feel it's relevant to his PS, which asks why he wants to be a doctor, then he doesn't need to include it. Technically, no one needs to include anything in their PS. I personally feel like the PS is a place to talk about your strengths, not your weaknesses. Many secondaries ask for an obstacle that you have overcome. I think this would be more apt there. Besides med schools don't really care about problems people had in high school, so there's nothing to apologize for.