The only way the person could have taken over his computer (that I know of) is if your friend had opened and exe or something. If your friend did open a program then it was most likely a rat (remote administration tool) which basically gives him control of everything on the computer, steals all passwords that were entered into the computer and can watch every thing he does ect. The safest way to get rid of a rat is to reinstall windows. If he didnt open an exe or anything then I dont know how the hell he got hacked. Hope this helps