Brigham refers to a member of the original Q12 who had a vision of the BOM plates and later apostatized.
Brigham Young, "Want of Governing Capacities Among Men—Elements of the Sacraments—Apostacy, etc.," June 5, 1859, in Journal of Discourses, 26 vols. (Liverpool: Amasa Lyman, 1860), 7:164
Some of the witnesses of the Book of Mormon, who handled the plates and conversed with the angels of God, were afterwards left to doubt and to disbelieve that they ever had seen an angel. One of the Quorum of the Twelve—a young man full of faith and good works, prayed, and the vision of his mind was opened, and the angel of God came and laid the plates before him, and he saw and handled them, and saw the angel, and conversed with him as he would with one of his friends; but after all this, he was left to doubt, and plunged into apostacy, and has continued to contend against this work. There are hundreds in a similar condition.
According to Matthew B. Brown, Plates of Gold: The Book of Mormon Comes Forth (American Fork, UT: Covenant Communications, 2003), 119–120n57, Brigham Young appears to be referring to Lyman E. Johnson.