@walkert Below are some resources for reaching out to Oneness Pentecostals. I thought the conversation starters from CARM were unique. Other pentecostals considered Branham to have gone off the rails during his own time, so it is clear that he strayed into serious error - some more information at the bottom.
What does your friend do with Matthew 28:19? It is utterly irrational to list them all at once if they are the same Person.
Matthew 28:19 - Therefore go and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit
Or what about Luke 4:1? If Jesus and the Spirit are the same, how could Jesus be filled with the Holy Spirit?
Luke 4:1 - Jesus, full of the Holy Spirit, left the Jordan and was led by the Spirit into the wilderness,
Perhaps the most important thing to remember when dialoguing with Oneness Pentecostals is to demonstrate to them the unconditional love and acceptance of Jesus Christ. The most problematic aspect of my theology when I was a Oneness Pentecostal was the belief that no one other than us Oneness Pentecostals was going to heaven. Trinitarian Christians simply were not saved! So every time I met Trinitarian Christians who clearly reflected the loving presence of Jesus in their lives by the way they related to me, I confronted more strong evidence that my theology could not be true.
A second vitally important component of witnessing to Oneness Pentecostals is to confront their misunderstandings of what Trinitarians believe. Like most Oneness Pentecostals, I was firmly convinced that Trinitarians worshipped three separate gods and that they didn’t “really” believe that Jesus Christ was Himself the Lord God Almighty. This is how Oneness Pentecostals are indoctrinated to perceive Trinitarians. Hence, when dialoguing with Oneness Pentecostals it is vitally important to be utterly empathic about your own belief that there is only one God — not three — and that Jesus Christ is the incarnation of this one God !
Conversation Starters with Oneness Pentecostals
If God is only one person, why did Jesus say in John 14:23, “If a man love me, he will keep my words: and my Father will love him, and we will come unto him, and make our abode with him.” If God is only one person, why does Jesus say, “we”?
Is Jesus His own Father?
If Jesus’ will and the Father’s will were identical, then why did Jesus express the desire to escape the cup but resigns Himself not to His own will but the will of the Father? See my article on this.
Critique of Branham
Branham believed Eve had children with the serpent in the Garden. This view led to racism and was something Branham taught. That should be a huge red flag for anyone that is following his teaching. Also, this belief sometimes led to a distortion of the nature of sin, emphasizing sin as fundamentally sexual rather than as rebellion against God / defining good and evil for ourselves. Branham also claimed to have received an angelic visitation - Muhammad and Joseph Smith claimed similar visitations. That in and of itself should be a warning.
Galatians 1:8 - But even if we or an angel from heaven should preach a gospel other than the one we preached to you, let them be under God’s curse!