Voices in the head

Yes, it is extremely easy. Just combine hypnosis with neuro-linguistic programming and perform "voice of command" hypnosis sequence.

A lot of people experience these "intrusive thoughts"

We live in a world of madness and a lot of us have been brainwashed by common media. It's tough but your friend needs to break away from addictive thinking patterns.

Whether your friend is addicted to drugs, alcohol, television, video games whatever it may be he probably just needs to find better balance in his life.
Moderation is key, but no easy task.

These ideas are often seen as intrusive thoughts, as some one else suggested. In actual fact they are the perception of ideas that are mentally presented in the Mind by someone closely related to the person. That person may be a relative or friends or work associate etc., but they sure are not a friend. Usually jealousy, wanting to gain power and influence in order to manipulate and control, wanting revenge or simply wanting narcissistic supply (getting pleasure from seeing someone else's pain and suffering) are the key motives.

I would advise your friend to watch my 5 videos on the Underlying Conditions of Disease. They describe the basic cheats of inhumane people, who are usually in the person's life. See my signature for the link.