LDS scholars have a real answer here. Say so before your friend does.
That the Father, Son and Holy Ghost are three separate beings, one in purpose but not one being. The Father and the Son have tangible bodies; see D&C 130:22 and the Gospel Topics page on the Godhead.
Joseph Smith—History 1:19 has God call all existing creeds "an abomination in his sight." Abraham 4:1 has creation carried out by "the Gods." And Smith preached a plurality of gods openly in the Sermon in the Grove, in June 1844.
Deuteronomy 6:4 — "The LORD our God is one LORD." Isaiah 44:24 is sharper still. God stretches out the heavens "alone" and spreads the earth "by myself." No council of gods.
1 Corinthians 8:4 says "there is none other God but one."
The Book of Mormon reads like the Bible here. Alma 11:28-29 asks whether there is more than one God, and answers no.
2 Nephi 31:21 says the same.
And this is the strongest set they have. First, John 17:22: Jesus prays that believers "may be one, even as we are one." Believers do not merge into one substance.
So the oneness in that same sentence reads as relational. Second, Jesus is visibly distinct from the Father all through the New Testament.
At his baptism. In Gethsemane.
In Stephen's vision at Acts 7:55-56. And 1 Corinthians 8:6 distinguishes one God the Father from one Lord Jesus Christ.
Third, respected Christian philosophers model the Trinity as three centres of mind in one essence and will. Alvin Plantinga, Richard Swinburne and William Lane Craig all do.
BYU scholars argue that lands near the Godhead. Fourth, Psalm 82 has plural elohim language native to the Old Testament.
Genuinely arguable, and the John 17 reading is fair. Do not pretend otherwise.
The difference shows where the doctrine goes further. Those philosophers still hold one uncreated divine essence.
Here there are three independent beings, two with bodies, at least one of whom became divine — and exaltation hands godhood to others. Isaiah 44:24 and 43:10 exclude that.
"Isaiah 44:24 says he made everything alone. Who else was there?"


Jesus prays we may be one as they are one, which reads as relational.
The strongest case runs on four fronts.
First, Jesus prays that believers 'may be one, even as we are one', at John 17:20-23. Believers cannot merge into one substance. So the oneness of Father and Son in the same sentence reads most naturally as relational. That is the 'one in purpose' reading.
Second, Jesus is visibly distinct from the Father all through the New Testament. At his baptism. In Gethsemane. In Stephen's vision at Acts 7:55-56. And 1 Corinthians 8:6 marks off 'one God, the Father' from 'one Lord, Jesus Christ'.
Third, respected Christian philosophers model the Trinity as three centres of mind united in essence and will. Alvin Plantinga, Richard Swinburne and William Lane Craig all do. BYU scholars argue that this lands close to Joseph Smith's Godhead. The quarrel, they say, is with Greek metaphysics added later, not with the Bible.
Fourth, Hebrew work on the divine council in Psalm 82 shows plural elohim language native to the Old Testament.
Arguable: the John 17 reading is fair, but Isaiah 44:24 says he worked alone.
The doctrine is documented, and a real conflict exists. But their case has real merit at exactly the point this entry tests. The John 17 reading is defensible. Social trinitarianism is a live position among Christian thinkers.
The clash gets sharp where this doctrine goes further. Those thinkers still hold one uncreated divine essence. Here there are three separate beings. Two have bodies. At least one attained divinity. And exaltation hands godhood to others. Isaiah 44:24 and Isaiah 43:10 shut that out.
Note the strain inside their own canon. The Book of Mormon reads as strict monotheism at Alma 11:28-29, against Smith's later Nauvoo teaching.
It is graded contested because the John 17 argument works on its own ground.