@wlittle6 I think the first step is to understand your friend’s journey. Why did he leave the Catholic Church? Why did he become a Unitarian? What does he believe now? People can be unitarian and believe quite a wide array of things.
You might try asking yourself some questions based on the Engel scale: Do I need to be cultivating or sowing in this season of my friend’s life? Is my friend even a seeker, or is he now more of a skeptic or spectator?
Praying that the Lord Jesus would grant you wisdom