In my humble opinion it is never a good idea to "force" someone to do something through spell craft. Say for example you force your friend to see a doctor by casting a spell on then he ends up going to the doctor - but not because he wanted to but because something made him go (like a car accident or some such event). You might try to cast a spell on him to make him more open minded to at least considering any psychological cause. Past trauma doesn't have to mean abuse. It can also include near death experiences or the loss of a loved one or witnessing a traumatic event. Like for example after 9/11 parents complained to news networks about replaying footage of the twin towers collapsing because it was giving their kids nightmares (true story). To adults and out developed brains we can understand that that one event was a single event, but to smaller children seeing the attack over and over again was like constantly reliving the event as though it had just happened. Its why kids can have nightmares after watching scary movies too.
But in my experience people who are resistant to change are resistant to magick and it is likely if you push the issue it will reach a breaking point where he gives you a bad reaction. Some such reaction might include not speaking to you or being verbally abrasive/argumentative. Chances are even if you cast some spell on him, he will still avoid the issue until he no longer can. It is an indication of delusional thinking to believe that holding your breath will prevent a "demon" from attacking you. While having that delusion doesn't mean anything serious, holding on to it really suggests this nightmare is being used as an inlet to avoid dealing with something. It could be your friend just has a hard time venting his emotions or expressing himself in a positive way. You can continue to press the issue if you wish and you might convince him to take action. But until he is really willing to help himself he will not likely every recognize the importance of the issue or that questioning and really dealing with the dream imagery is the only way to get over it.
Its kind of like putting your hand on a hot stove. The first time you do it, it really hurts. You learn to not put your hand on a hot stove. But the trauma of the first event is so great that now you are afraid to even go near the stove. Every time you see it, it evokes fear. So you avoid it and deal with the trauma by doing something that doesn't hurt. The demon in his dream represents something unpleasent, thats simply the case. By holding his breath, he is avoiding this unpleasant thing. In fact breathing has its own symbolism. Dreaming that you are holding your breath can mean tension and anxiety or being unrelenting in your ways. Breathing can relate to life and relaxation. So if you are holding your breath in your dream, you are avoiding life and not relaxing.
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