Is my friend considered to be among those about whom Allah Almighty said: "Nor is there blame upon the insane," after he felt a strong urge towards immoral acts, and later discovered that he was afflicted by jinn possession? And if not, what should he do after committing some major sins like fornication?

The desire to commit forbidden acts stems from Satan, the self that incites to evil, and a weak religious deterrent. A person who is accountable is held responsible for their actions as long as they are sane, and the pen is lifted from the insane until they recover. There is no validity to the words of sorcerers. One must verify before attributing a word to the Book of Allah. And it is obligatory upon anyone who has committed sins to offer sincere repentance, for Allah accepts repentance and forgives all sins for those who repent.