The pitching motion is very damaging to multiple parts of the arm and shoulder. The muscles get torn up pretty bad, thus the graduated requirements for rest based on how much you throw. The damage gets worth with every pitch. Continuing to throw after the muscles are that torn up will cause serious damage. It's different for everyone but there is a reason MLB pitchers rarely if ever pitch past 110 pitches, and they are grown men who have received the best training and prep there is. 150 is insane. The damage done likely won't show up immediately, but it's likely it won't heal before he throws again, and continue to get worse until he has a major injury.