Callista

Both abuse and oxygen deprivation can predispose someone to mental illness; but the main factors are usually genetic.

I think your friend probably needs to talk to a psychiatrist. It makes sense that you're scared for them; but there are a lot of resources out there to help people think more clearly, take control of their lives, etc.

What's most worrisome is the existence of this "violent and destructive personality". Whatever it is, it's not good--and could end up hurting your friend or someone else. If you can convince your friend that a psychiatrist could help them gain control of this aspect of themselves, then do so. (This would indeed be a major focus of therapy, since it is the most immediate threat.) In any case, make sure that your friend isn't alone--that there's someone nearby that they can go to in case of trouble.

It's possible to see spirits without having any mental problems at all. Many religions include such visions, and the people who have them are usually quite normal, mentally. If your friend has a background that includes cultural or religious expectations that s/he might see visons of this sort, then it could be quite normal.

However, the emotional disturbances and the "multiples" your friend reports are matters of concern and should be discussed with a doctor; and if s/he is actually hallucinating, then that is also a problem to be addressed.

A good way to start is to go to your family doctor. He can judge whether the symptoms are serious enough for a psychiatrist (in my opinion, they are); and can refer your friend to a specialist who can help with that particular type of problem.