release a demon

Maybe the reason why it is so difficult to remove is because it is self created. They don't call it "inner demon" for nothing. You don't "remove" such a thing, you face it and deal with it like any other emotional/mental obstacle in your life. It gets stronger the more you run from it or consider it a separate entity. You give it power that way through fear and giving it independence.

Meditation will help to narrow down what the root of the destructive inner force exactly is which will allow you or your friend to tackle it with whatever way you wish, self help books, counseling, guided meditation, mantra, etc.

Maybe the difficulty is in seeing this "being" as something separate to begin with. It is you. Once they realize that, they will realize they have control/power over it. Many say that guides are internal as well, even though we can experience them in our environment. The negative parts can do the very same. They don't often unless they become very strong. So I would guess this person has quite a few serious issues in their life, something that has spawned perhaps quite a few destructive emotions--fear, hate, guilt, deception, jealousy, etc.

Time to turn within and get that worked out. Good luck!

I think, there is not any demon. Demons are higher spiritual beings and they anyone not possess.
It may be an black magick, or lower spiritual being as larvae, phantasms.... and similar. See by Franz Bardon, Initiation Into Hermetics.
There is helpull when one learns to meditate alone. When enhance his own mental powers.