I have had friends go nuttty too. Most of the time it is due to an excesive amount of halucinogens or dark liquer and it goes away as soon as all of the drug is out of your body. I only have one friend who went coo coo for coa coa puffs and never came back to reality, but that was because she did A LOT of different hard drugs. If your friend took dxm for a week straight, there are going to be chemicals in his body for a minute, even though isnt that just a cough syrup?. It also depends a lot on how his hormones mixed with dxm and HOW MUCH dxm he took at a time (enough to trip balls?). hopefully his damage is reparable. It also kind of sounds like he has bipolar disorder. This disease usually doesnt start till your late teens and you MUST get medication b4 you hurt someone . My advice for you would be to be very very very nice to him and try to stay away for your own safety....also it sounds like his parents are just as crazy as him. I have smoked everyday for the past 4 years and if anything, I am a lot more sane than I used to be.
bottom line....kids need to be more smart when it comes to using drugs. Some people can handle it, some cant. Just remember, every chemical you put in your body will leave a mark in your brain, liver, lungs, heart, etc.