You really need to provide more detail about your friend's situation. What was he like before LSD? Was he certain what he ingested was LSD? What happened to him while tripping? Did anyone else trip with him? Were you his trip sitter, or did you speak directly with them before and after the experience? I've seen someone act crazy, or "retarded" while tripping, but in years of reading clinical data, experience reports, and collecting a good about of personal experience on the subject I've never heard someone describe a pronounced drop in IQ after taking real LSD.
As alllium was saying, there is a statistical correlation between LSD use and the emergence of schizophrenia in people with a family history of mental illness. Personally, I've never quite concluded "causation" from this correlation though. The fact is that schizophrenia tends to appear around the same time when most people start experimenting with drugs (mid-teens to early twenties). It may be that LSD brings this to the surface quicker, but it may be that these people end up crazy because they were going to end up crazy weather they used LSD or not. It may not be something that you want to risk, though, if you have a family member who suffers form schizophrenia.
Hope this is helpful, but I'd really be interested in more information on your friend.