Jamieson, Fausset and Brown on Job 16:21

21. one--rather, "He" (God). "Oh, that He would plead for a man (namely, me) against God." Job quaintly says, "God must support me against God; for He makes me to suffer, and He alone knows me to be innocent" [Umbreit]. So God helped Jacob in wrestling against Himself (compare Job 23:6; Ge 32:25). God in Jesus Christ does plead with God for man (Ro 8:26, 27).

as a man--literally, "the Son of man." A prefiguring of the advocacy of Jesus Christ--a boon longed for by Job (Job 9:33), though the spiritual pregnancy of his own words, designed for all ages, was but little understood by him (Psa 80:17).

for his neighbour--Hebrew, "friend." Job himself (Job 42:8) pleaded as intercessor for his "friends," though "his scorners" (Job 16:20); so Jesus Christ the Son of man (Lu 23:34); "for friends" (Joh 15:13-15).