We understand that you are concerned about ###'s health and his adherence to a vegan diet, which may be contributing to his mental health issues due to a lack of vitamin B12. We must emphasize that our prayers should seek God's will and ###'s overall well-being, rather than merely focusing on a change in diet.
To find the proper biblical approach to diet restrictions we look to Mark 7:14 for guidance. Mark 7:14 And he called all the multitude to him, and said unto them, Hearken unto me every one of you, and understand: 15 There is nothing from without a man, that entering into him can defile him: but the things which come out of him, those are they that defile the man.
The most important this scripture points out is that no outside thing can enter us and defile us. However, this doesn't mean the food we eat isn't important. Both a plant only diet and a diet low or lacking in essential nutrients can lead to poor health. There is not one diet that is set by our Lord that we have to follow. Realize that gluttony and frequent drunkenness are identified in the bible Proverbs 23:2 and Proverbs 23:20. It is important to look at what our body needs and what is needed physically to flourish in the our each situation.
From reading in texts concerning B12 a lack of B12 may cause 'subacute combined degeneration of spinal cord. This deficiency can cause brain and nervous system damage which may cause lasting damage. A person can become severely disabled and unable to move.
We, however even with these warnings, knowledge and biblical insight, must ultimately remember to place our faith in God's power to heal and guide, rather than just in dietary changes and trying to manipulate outcomes. We would advise for ### also considering a biblically based counseling. We also need to understand that biblical based counseling recommends that medications be avoided when they are dangerous or have long term adverse effects. However, they can be used when needed if there aren't cheaper, better, safer, and mentally healthier alternatives, but that is a conversation for another time as a group and with a doctor and counselor.
Let us pray for ### with a focus for our Lord to influence and direct his mind properly and decisions to get him to a place where he is desiring happiness, health, and sobriety in his life. Let us also pray and thank God for his sovereignty as we pray in alignment with His purpose and well being for ### in Jesus' name along with any given spirit that condemns or misdirects ###. It is unfortunate that there is so much confusion in the world and misdirection by other party. It can be so hard to set aside pride and make a step towards bettering oneself in a step towards God. Let us pray in align with our Lord for His will and for our brother in faith to overall be well in Jesus' name. Our most holy and merciful God we ask to give us your grace in everything we have mentioned.
In Jesus' name we pray that ### will surrender his situation to God, trusting in His divine plan. We pray that ### will be receptive to the advice of medical professionals and make wise decisions regarding his health. We also pray that ### will seek God's wisdom and guidance in all aspects of his life, including his diet, and that he will be open to making changes that honor his body as a temple of the Holy Spirit Romans 12:1 Therefore, I urge you, brothers, in view of God's mercy, to offer your bodies as living sacrifices, holy and pleasing to God--this is your spiritual act of worship.
We encourage you to continue praying for ###, trusting that God is at work in his life. Philippians 4:6 In nothing be anxious; but in everything by prayer and supplication with thanksgiving let your requests be made known unto God.
Amen! "Again, truly I tell you that if two of you on earth agree about anything they ask for, it will be done for them by my Father in heaven" (Matt 18:19). Trusting God's intervention for all these needs. Keep praying, warriors! In Jesus' name! 
