We stand with you in prayer for this individual who is facing mental distress. We first acknowledge that all healing and deliverance comes from God through faith in the name of Jesus Christ. It is by His authority that we can approach God as it says in John 14:6 (WEB), "Jesus said to him, 'I am the way, the truth, and the life. No one comes to the Father, except through me.'" We need not be afraid because He has given us power and authority over all works of the enemy. Be reminded of 1st John 4:4 (WEB) "You are of God, little children, and have overcome them; because greater is he who is in you than he who is in the world."
We pray that God will heal this person's mind and provide him peace that transcends all understanding as it says in Philippians 4:7: "And the peace of God, which surpasses all understanding, will guard your hearts and your thoughts in Christ Jesus."
However, we need to clarify that in the Bible, the use of the term "demented" is not found. Instead, scripture speaks of those who are afflicted, troubled, or under oppression. We are encouraged to pray for those who are facing difficulties such as these.
As for deliverance, it is something to be addressed with guidance from the Holy Spirit as we see in Acts 10:38 (WEB), "even Jesus of Nazareth, how God anointed him with the Holy Spirit and with power, who went about doing good and healing all who were oppressed by the devil, for God was with him."
As for the prayer for protection, we agree with you in Ephesians 6:16 (WEB), "above all, taking up the shield of faith, with which you will be able to quench all the fiery darts of the evil one."
We pray for all this in Jesus Christ's mighty name, believing that God will hear and answer our prayers according to His will. We continue to lift this person up to God, trusting in His healing and deliverance.