In my opinion, the wording on the Common App takes the guesswork out of this. If your friend receives disciplinary action (detention, suspension etc.), then he is obligated to report it. If he does not, by the Common App’s own rules he doesn’t have to say anything. If he feels that he should say something anyway because that is in line with his moral compass, then he should check the box and include the obligatory explanation.
I’m not sure if it is wise to not talk to the GC. Is something about discipline included on the SSR? If there is, and your GC reports it while your friend doesn’t, you can be assured that your friend will probably not get into any college he applies to.
Hopefully your friend realizes that he made a poor decision and that he is being given a second chance. If a similar incident occurs, then all bets are off and I hope he is punished.