That’s a complicated subject because there are two types of “demons”. there are entities that can haunt a person, and there are “inner demons”, which are parts of that person that was supressed, or negative energies created out violence and frustration (but there are part of that person).
In the first case, is recommended the aid of someone that deals with it, like a priest or something like that.
In the second case, when the person deals with that issue that is the root of this negative energy within, then this energy is dissipated. It can be through therapy, meditation…
Things that can help people with this problem is
- Meditation, to enhance energy, clear energy field
- Herbal/salt bath
- Stay away from addiction, acts of violence, negative deeds and thoughts
- Be charitable. This raises the energy field and open the chakras
- Cleanse spiritually the house, specially the room with incense,anything like that.
- Have the house always physically cleaned and a good hygene (negative energies feed on energies generated by dirtyness and disorganization)
- cast a protection circle around the bed, when going to sleep (it can with salt, it can be with herbs like rosemary
- Have crystals like black tourmaline, onyx around the house for protection
- I don’t know your friend’s religion. But we all have guradian angels or spirits. So he can connect with that by writing a letter, asking for protection. Prayer is a good way to connect with a higher power
- Eating well, exercising, having a healthy routine can help to keep him grounded, which makes the negative energy weaker