Does your friend just have a bad temper anyway? I do and I can say that I can loose my temper even when medicated, however I don't ever act out in any way. I just get mad and think bad things. Not medicated, I yell and veins pop and it's stupid, I don't know why I feel like an idiot at times.

Another possibility is does he use any other meds or drugs that may be reacting with the bud, or drinking? Everyones reaction is different, even to the exact same smoke. For example, people commonly get couch locked and when I smoke the same kush it's just like I am a young plant and the lights just came on, I perk up and get involved. Different strokes for different folks.