We understand your concern for Brian and the situation you've described. Let's bring this before the Lord in prayer and seek His guidance according to His Word.
Firstly, we must remember that Jesus declared all foods clean for those who have faith (Mark 7:19 WEB), and it is not what enters into the mouth that defiles a man, but what proceeds out of the mouth (Matthew 15:11 WEB). Therefore, it's not about the consumption of meat or vegetables, but the heart and faith behind it.
However, we must also address the issue of causing others to stumble. In Romans 14:20-21 (WEB), it is written, "Don’t destroy the work of God for the sake of food. All things indeed are clean, however it is evil for that man who creates a stumbling block by eating. It is good to not eat meat, drink wine, nor do anything by which your brother stumbles, is offended, or is made weak." Brian's insistence on a vegan diet and his actions towards others seem to be causing division and strife, which is not in line with God's will.
Let us pray:
"Dear Heavenly Father, we come before You in the name of Jesus Christ. We lift up Brian to You, asking that You reveal to him the truth about dietary choices and faith. Lord, if Brian is indeed promoting a vegan diet as a result of the influence of Yiguandao, we pray that You cut off this influence and bring him into the full knowledge of the truth in Jesus Christ. Help him to understand that it is not about what we eat, but about our faith in You.
We also ask, Lord, that You convict Brian's heart about causing others to stumble. Help him to see the error of his ways and to turn from them. Give him a heart of compassion and understanding, instead of one that causes division and strife.
Lord Jesus, we know that it is only by Your name that we can be saved. If Brian has not fully accepted You as his Lord and Savior, we pray that You reveal Yourself to him and draw him to You.
In Jesus' name, we pray. Amen."
We encourage you to continue praying for Brian, that he may come to a full understanding of the faith and walk in a manner worthy of the Lord, fully pleasing to Him, bearing fruit in every good work and increasing in the knowledge of God (Colossians 1:10 WEB).