it appears that K2 is currently illegal and considered potentially dangerous. they recently raided some shops that were selling it.
http://www.post-gazette.com/stories/local/region/authorities-raid-shops-selling-k2-646150/
the use of that drug is strongly discouraged even by one of its early developers, they say it's dangerous but I can't find any of the side-effects you refer.
http://www.livescience.com/6149-fake-weed-real-drug-k2-causing-hallucinations-teens.html
If it's been a month already it isn't a medical emergency. he should see a Neurologist, if he's a minor he'll have to talk about it with his parents. If he'll go to the ER they will obviously notify his parents eventually. so either way he'll have to tell his parents or just keep the symptoms.
You could also mail or call the CDC Center for disease control and ask for advice or fill out a form on the US Food and Drug Administration website.
There is something else that you must consider: there is a good chance that smoking K2 has nothing to do with his symptoms. it is extremely common for people to inappropriately attribute the cause of their symptoms to something they ate, drank or smoked.
What is more important your health, which might be treatable, or possible legal reprocussions. See a Doctor!
i have to say i dont know what k2 is,i assume its some sort of narcotic as you say he smoked it..and smoking something even cigs isnt good for ye..but without a doubt,if your concerned then convince your mate to get it checked out..always best to safe than sorry,..and like tahjahani said..you wont be prosecuted..understandably u may be asked questions..and thats even maybe..but i think your mates health is of more concern..hope he ok and hope you do the right thing good luck
I'm unfamiliar with k2 but if he is experiencing side effects he should go to the hospital and tell them what he had because you do'nt want him toaffect his health permanetly and as for legal issues it wa a month ago they can't prosecute for something you did over a month ago