Lower realms rising?

This person could probably benefit from modern psychiatric treatment. Unfortunately Buddha said it's impossible for insane people to make progress in dharma (stream enterer or higher). Buddha didn't give detail on this subject. I don't think Buddha ever talked about lower realms raising but he did talk about devil (mara) both literally and figuratively. Meditation in first jhanna is enough for one to temporary elude from mara or dangers from sensual pleasures and anger.

an unnatural state of mind which his friends first assumed to be a typical p**n addiction

I can't off the top of my head list symptoms of "a typical p**n addiction". So I'm not sure there is such a thing: could it be like saying, "This sour milk is a typical witch infestation"?

Anyway, there's an English verb "to demonize" that might be relevant:

portray as wicked and threatening.
"seeking to demonize one side in the conflict"

Some examples a "demonizing" p**n (in that sense of the word "demonize") might include saying:

• Porn is wicked
• You'll go to hell if you watch p**n
• People who like p**n are (or become) evil
• Porn is addictive
• You are a p**n addict

Some alternative statements which are, IMO, kinder:

• Porn is unsatisfactory
• There are other, more satisfactory states
• While you watch p**n, you don't learn how to be alone with yourself, and/or to be with others

In summary I suggest you avoid "demonizing" people and their activities.

Here for example is a (non-Buddhist) set of mottos which helped to guide preschool teachers: Children Learn What They Live

If children live with criticism, they learn to condemn.
If children live with hostility, they learn to fight.
If children live with fear, they learn to be apprehensive.
... etc. ...

What can we take as reference from the teaching to understand this

There are some references to possession in the Vinaya; for example, on page 308 of The Patimokkha Rules Translated & Explained by Thanissaro Bhikkhu

Raw flesh and blood are allowed at Mv.VI.10.2 only when one is possessed by non-human beings. Thus, in more ordinary circumstances, one may not eat raw fish or meat even if of an allowable kind.

There's some mention of exorcism:

The Vibhanga states that bhikkhu who kills a “non-human being”—a yakkha, naga, or peta—incurs a thullaccaya. The Commentary adds a devata to this list, and goes on to say that a spirit possessing a human being or an animal can be exorcised in either of two ways. The first is to command it to leave: This causes no injury to the spirit and results in no offense. The second is to make a doll out of flour paste or clay and then to cut off various of its parts (!). If one cuts off the hands and feet, the spirit loses its hands and feet. If one cuts off the head, the spirit dies, which is grounds for a thullaccaya.

Also Appendix I of Part 2 (Khandhaka Rules) mentions insanity as a reason why a monk may or may not be present at a community meeting (i.e. the meeting may be held with or without them). I don't know how/whether monks distinguish between "possession" and "insanity".

The Ratana Sutta and the Atanatiya Sutta are examples of suttas which describe how to pacify demons.

See also The Book of Protection - Paritta:

The Book of Protection which is an anthology of selected discourses of the Buddha compiled by the teachers of old, was originally meant as a handbook for the newly ordained novice. The idea was that those novices who are not capable of studying large portions of the "Discourse Collection" (sutta pitaka) should at least be conversant with the Book of Protection.

What would be the proper cause of action other than precepts?

Metta.

Here is a story attributed to Ajahn Chah:

There was once a woman in Thailand who was said to be posessed. She was violent: she'd attack anyone and everyone. She never cleaned herself. She screamed and cursed at passers by. One day, the villagers went to see Ajahn Chah, hoping to get him to exorcise the demon. Ajahn Chah hemmed and hawed, and the villagers persisted till he said, "fine...bring her by tomorrow."

The next day, the villagers brought her to the monastery. Ajahn Chah had some monks dig a hole, and told another one to go boil some water. So off they went. They held the woman down in the hole, and he kept shouting for the monks to hurry with the water. When they came back with a steaming kettle of boiling water, he said, "Okay...we have to pour the water on her. That will get the demon out." They held the kettle over the woman as if to pour it, and the woman stopped her behavior and calmed down. He was able to talk to her about her problems and help her from that point.

People confuse mental illness with demon posession. Ajahn Chah knew she wasn't posessed. It wasn't demons. She was in horrible pain and needed help, but no one could help her as long as she was acting the way she did. People who are "posessed" need professional help, not an exorcism.

That's not to say that even "professional help" is necessarily effective. Mental illness is difficult to treat. All the more reason for metta, IMO.

Still at the moment I think of it as a form of wrong view: for example, thinking "I am me" (or whatever it is that people think) is a form of identity-view. Similarly, thinking "I am human" or "I am a supreme being" or "I am a demon" are also, IMO, apparently examples of identity-view.

If you're talking with someone who is expressing that kind of insanity I think you're supposed to avoid confirming their delusion; for example

• Saying "No you're not a demon" can be counter-productive because it's contradicting them (how would you like it if I contradicted you -- if you came up to me an said, "Hi there, I'm Theravada" and I replied "No you are not"?).

  This is based on theory that saying "I am a demon" is trying to communicate something. Whatever it's trying to communicate, it's not communicating it well/clearly, but that's their 'personal truth' and it's not good enough to simply deny what they're trying to say. It's miscommunication, but you shouldn't contradict (i.e. deny that there was any communication at all). So instead of denying, you might reply something like, "Are you telling me that you're feeling bad?"

• Saying "Yes you are demon" can be counter-productive because that confirms their delusion, and is I think an expression of a wrong/reifying view whereas you ought to be offering them something gentle and real (how would you like it if I came up to you and said, "Hi, you're a demon"?).

Even if you do meet a demon I think that metta (and, maybe, faith: faith that the Buddha and Dhamma can help whatever ails them) is the proper course of action.

If the man was a monk, he would eventually be expelled from the monastic order. Similarly, if the man's mind has been consumed by p**nography to the point his behaviour is contrary to social norms, he should probably be forbidden from entering the temple. Often, people with addictions cannot be helped, until those people can recognise for themselves they have a serious problems.

The four brahma vihara (metta, karuna, mudita, upeka) include equanimity when we wish to help someone but are unable to.

In short, the monks who are responsible for the temple should probably telephone the police to remove the deranged man from temple. Buddhism does not have the power to save everyone.